Get it here.
From the version history, changes in 5.1:
* New: CodingTechnologies AACPlus encoder! 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