|Operating System:||Microsoft Windows, Linux (WA3 only)|
Winamp was first released by Justin Frankel in 1997. Current Winamp development is credited to Ben Allison (Benski) and Maksim Tyrtyshny. Winamp grew from 33 million users in February 2005 to over 57 million users in September 2006.  
Besides MP3, Winamp supports a wide variety of contemporary and specialized music file formats, including MIDI, MOD, MPEG-1 audio layers 1 and 2, AAC, M4A, FLAC, WAV and Windows Media Audio. Winamp was one of the first common music players on Windows to support playback of Ogg Vorbis by default. It supports gapless playback for MP3 and AAC, and Replay Gain for volume leveling across tracks. In addition, Winamp can play and import music from audio CDs, optionally with CD-Text, and can also burn music to CDs.
Winamp supports playback of Windows Media Video and Nullsoft Streaming Video. For MPEG Video, AVI and other unsupported video types, Winamp uses Microsoft's DirectShow API for playback, allowing playback of most of the video formats supported by Windows Media Player. 5.1 Surround sound is supported where formats and decoders allow.
Winamp supports many types of streaming media. Its SHOUTcast technology provides free access to Internet radio and Internet television, and Winamp can also access XM Satellite Radio, and AOL Video content. It can access the Singingfish audio/video search engine. Winamp can also be used as an RSS media feeds aggregator capable of displaying articles and playing streaming media. SHOUTcast Wire provides a directory and RSS subscription system for podcasts. 
Winamp has extendable support for portable media players. Device plugins are currently included for iPods and Creative NOMADs, Mass Storage Compliant devices, and devices that support the Microsoft PlaysForSure and ActiveSync technologies, such as those running Windows Mobile.
In more recent versions of Winamp, an extension has been added to allow users to share their media libraries to their gaming consoles on the same network through Winamp Remote. This was created through a partnership with Orb. The same extension allows users to access their media libraries anywhere via internet connections.
The Winamp software development kit allows software developers to extend Winamp's functionality through the use of plug-ins, which are categorized into the following seven types:
Input plug-ins allow Winamp to play additional media formats. Easy development of specialized Input plug-ins contributed to Winamp's versatility compared to monolithic media players. For example, popular video game music has driven development of plug-ins to play back game console music files, such as NSF, USF, GBS, GSF, SID, VGM, SPC, PSF and PSF2.
Other plug-ins included in the installer bundle activate features such as global hotkeys. Users can enable these plug-ins by selecting them during installation (a feature made possible by Winamp's use of NSIS to package the application).
A wide variety of plug-ins are available on the Winamp web site.
Skins are aesthetic revisions of the Winamp graphical user interface (GUI). Winamp has published documentation on skin creation, and invites contributors to publish skins on Winamp.com. As a result, freely available Winamp skins now number in the thousands.
Winamp 5 supports two types of skins - "classic" skins designed to Winamp 2 specifications (static collections of bitmap images), and more flexible, freeform "modern" skins per the Winamp 3 specification. Modern skins support true alpha channel transparency, scripting control, a docked toolbar, and other innovations to the user interface, but many skins found on Winamp.com remain committed to the "classic" skin specification. Online communities of skin designers, such as 1001 Winamp Skins and DeviantArt, and the active forums on Winamp.com, attest to the popularity of the feature and its flexibility as a medium for creative expression. As the number of independently produced works has increased, genres or categories of skins have emerged. Promoting celebrities, fashion models, films, cars, bands, brands, and other forms of entertainment remains a common staple of the medium. Artists have also created designs for their own sake - parodies of other interfaces, nostalgic emulations of old hardware and operating systems, hand drawn art, 3-dimension renderings employing transparencies, minimalist and high contrast designs, and clever implementations of vector graphics.
The default (classic) skin of Winamp was designed by Nullsoft employee Steve Gedikian.
The minimalist WinAMP 0.20a was released as freeware on 21 April 1997. Its windowless menubar-only interface showed only play (open), stop, pause, and unpause functions. A file specified on the command line or dropped onto its icon would be played. MP3 decoding was performed by the AMP decoding engine by Tomislav Uzelac, which was free for non-commercial use. The acronym "AMP" stood for "Advanced Multimedia Products". Justin Frankel and Dmitry Boldyrev integrated this engine with their user interface. WinAMP 0.92 was released as a freeware in May 1997. Within the standard Windows frame and menubar, it had the beginnings of the "classic" Winamp GUI: dark gray rectangle with silver 3D-effect transport buttons, a red/green volume slider, time displayed in a green LED font, with trackname, MP3 bitrate and "mixrate" in green. There was no position bar, and a blank space where the spectrum analyzer and waveform analyzer would later appear. Multiple files on the command line or dropped onto its icon were enqueued in the playlist.
Version 1.006 was released June 7, 1997 renamed "Winamp" (lower case). It showed a spectrum analyzer, and color changing volume slider, but no waveform display. The AMP non-commercial license was included in its help menu.
According to Tomislav Uzelac, Frankel licensed the AMP 0.7 engine June 1, 1997 Frankel formally founded Nullsoft, Inc. in January 1998, and continued development of Winamp, which changed from freeware to $10 shareware. In March, Uzelac's newly founded company, PlayMedia Systems sent a cease-and-desist letter to Nullsoft, claiming unlawful use of AMP. Nullsoft responded that they had replaced AMP with Nitrane, Nullsoft's proprietary decoder, but Playmedia disputed this.
Version 1.90, released March 31, 1997 was the first release as a general-purpose audio player, and documented on the winamp.com website as supporting plugins, of which it included two input plugins (MOD and MP3) and a visualization plugin. The installer for Version 1.91, released 18 days later, included wave, cdda, and Windows tray handling plugins, as well as the famous Wesley Willis-inspired DEMO.MP3 file "Winamp, it really whips the llama's ass".
Winamp 2.0 was released on September 8, 1998. The 2.x versions were widely used and made Winamp one of the most downloaded pieces of software for Microsoft Windows. The new version improved the usability of the playlist, made the equalizer more accurate, introduced more plug-ins and allowed skins for the playlist and equalizer windows.
PlayMedia Systems filed a federal lawsuit against Nullsoft in March 1999. PlayMedia was granted an injunction against distribution of Nitrane by Nullsoft, and the same month the lawsuit was settled with out-of-court licensing and confidentiality agreements. Soon after, Nullsoft switched to an ISO decoder from the Fraunhofer Gesellschaft, the developers of the MP3 format. Nullsoft was bought by AOL in June 1999 for US$80 million.
Nullsoft relaunched the Winamp-specific winamp.com in December 1999 to provide easier access to skins, plug-ins, streaming audio, song downloads, forums and developer resources.
As of June 22, 2000 Winamp surpassed 25 million registrants.
The next major Winamp version, Winamp3 (so spelled to include mp3 in the name and to mark its separation from the Winamp 2 codebase), was released on August 9, 2002. It was a complete rewrite of version 2, newly based on the Wasabi application framework, which offered additional functionality and flexibility. Winamp3 was developed parallel to Winamp 2, but many users found it consumed too many system resources and was unstable (or even lacked some valued functionality, such as the ability to count or find the total duration of tracks in a playlist). Winamp3 had no backward compatibility with Winamp 2 skins and plugins, and the SHOUTcast sourcing plugin was not supported. No Winamp3 version of SHOUTcast was ever released.
In response to users reverting to Winamp 2, Nullsoft continued the development of Winamp 2 to versions 2.9 and 2.91. The beta versions 2.92 and 2.95 were released with the inclusion of some of the functionality of the upcoming Winamp 5. During this period the Wasabi cross-platform application framework and skinnable GUI toolkit was derived from parts of the Winamp3 source code. For Linux, Nullsoft released an alpha version of Winamp3 in October 9, 2001 but has not updated it despite continued user interest.
The Winamp 2 and Winamp3 branches were later fused into Winamp 5 — Nullsoft justified their non-sequential christening by quipping that 2 + 3 = 5 — taking the best parts from both applications. They also joked that "nobody wants to see a Winamp 4 skin" ('4 skin' being a pun on foreskin). It was also joked that "Winamp 5 is so good they skipped a number." Winamp 5 was based on the Winamp 2 codebase, with several Winamp3 features (e.g. modern skins) incorporated. Winamp 5.0 was released in December 2003. Most of the Wasabi framework built for creating Winamp3 and its components was released as open source, and as of 2005 an active development effort has succeeded in making a standalone version of Wasabi, minus the skinning and scripting modules which were never released.
Winamp 5 comes in three versions. Lite and Full are freeware, and Pro requires registration and () sells for US$19.95. The Lite version has far less functionality (largely supplementable with plugins) while still replicating most of Winamp 2's feature set in a far smaller installer. The Full version offers a richer feature set, including music ripping and CD burning at limited speeds (6x for ripping and 2x for burning). The Pro version features unlimited speed music ripping and CD burning and MP3 encoding.From version 5.2 on, support for synchronizing with an iPod is built-in.
Winamp 5.5: The 10th Anniversary Edition was released on October 10, 2007, ten years after the first release of Winamp. (A beta preview had been released on September 10, 2007.) New features to the player included album art support, much improved localization support (with several official, localized Winamp releases, including German, Polish, Russian and French), and a unified player and media library interface skin. This version also dropped support for Windows 9x.
Version 5.5 displays the classic Winamp 2 interface when the Windows Aero taskbar preview is seen, instead of displaying the original window contents, as other programs do. There is a workaround, but it works only when Winamp is not minimized.
Winamp has always included a number of Easter eggs, hidden features that are accessible via undocumented operations.
Here are two examples of Easter eggs included in Winamp 5 with the "modern skin" enabled:
With the default (Modern) skin chosen, stretch the main window until the Beat Analyzer appears; it says "BEAT" under it. Hold down Ctrl+Alt+Shift and click exactly at the center of the Beat Analyzer, then play a song with fast beats and loud bass.
While holding Ctrl+Alt+Shift type "nullsoft" in the main window. This must be typed in lowercase, (nullsoft) and not uppercase (NULLSOFT). Play a song with beats and loud bass to enjoy the effect.
In older versions of Winamp, with the default Base Skin enabled, doing this would toggle the top bar text between WINAMP and "IT REALLY WHIPS THE LLAMA'S ASS!"