It's Windows only for now.

The architecture of Winamp is made of various plugins. WACUP is replacing them bit by bit.

Once everything is replaced then porting could be possible, though it's been only built for Windows so there must be a lot of Windows-isms in there.

There have been Linux-based clones of Winamp for a long time, such as XMMS (which directly supports Winamp skins).

5.666 installed and is running well under wine for me.

Weird? It's Winamp lol