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.
Some anonymous do-gooder has made an older version (it updates itself to 0.6.6.10) available right here. That one works fine on my Powerbook G4 PPC running 10.5.8. Contains all the main features - it's from September 2011, I believe.