Transmission 2.31 for Tiger can still be found on the
Filezilla, a full featured FTP client, has a page of Tiger builds here.
Handbrake 0.9.5 for Leopard PPC is here.
Songbird for Tiger and Leopard PPC is still hosted at Thomas Legg's Geek Blog here.
A universal binary for the racing simulator TORCS courtesy of Stephen Hudson is in my Mediafire folder. It's for v1.3.0 (the last official Mac build was for 1.2.4).
zDoom, an enhanced port of the original Doom, has Mac builds on this page. The latest stable release is 2.5.0 at the bottom. You need the original game files to play this as it's just an engine.
And of course there's TenFourFox and TenFourBird, though they might be considered forks rather than unofficial builds. And also there's Leopard-Webkit for those of you who want a webkit browser.