How can I run Spotify on a PowerPC Mac?
This site appears to have a link to the latest version that supports PPC.  I don't have a PPC Mac, so I can't verify it.  

It also mentions that this will not last forever, as Spotify will update their servers at some point and not allow that version to connect any more.  



You can also try Spotify-0.6.6.10.dmg from PowerPC Software Archive.