Nullsoft Winamp is a fast, flexible, high-fidelity media player for Windows. Winamp supports playback of many audio (MP3, OGG, AAC, WAV, MOD, XM, S3M, IT, MIDI, etc) and video types (AVI,ASF,MPEG,NSV), custom appearances called skins (supporting both classic Winamp 1.x/2.x skins and Winamp 3 freeform skins), audio visualization and audio effect plug-ins (including two industry dominating visualization plug-ins), an advanced media library, Internet radio and TV support, CD ripping, and CD burning. Changes:

• New: CodingTechnologies AACPlus encoder! The MP3 Killer! Awesome at low bitrates too!
• New: Grand Prize Winner of the Internet Surround Music Project! KAJE - Hey Buddy (featuring Afrika Bambaataa).
• New: SHOUTcast Wire - A Media RSS Browser and Subscription Service.
• New: Predixis Smart Playlist Generator (Mix your tracks based on what they sound like!).
• New: AOL Radio featuring (((XM))) Free Sampler!
• New: CD Ripping now available at 8x in free version!
• New: Microsoft WMA encoder.
• New: Over 200 new Milkdrop presets!
• Update: in_vorbis now returns bitrate in getextendedfileinfo.
• Fixed: WMA stuttery playback.
• Fixed: out_ds copy button on status tab now works all the time.
• Fixed: Much improved ripping and playback with Sonic engine.
• Fixed: Video window now resizable regardless of Sidecar.
• Fixed: Length of long mp3`s now reported properly.
• Fixed: Silent install switch now works properly.
• Fixed: Simultaneous CD Playback and Ripping.

Information >> Official Site
Download >> WinAmp 5.1 Pre-Release (Beta 3) (mirror added)

Update: Pre-release information or why 5.1 has been removed for download from the official servers.