Seeing as you can't bring it any higher than OS X 10.5, I'd probably recommend dropping a Linux or *BSD distro on there (unless you're specifically looking to learn or use OS X). Leopard was released in 2007, and the Intel transition started happening in 2006, so those two factors are working against you big time.
As far as supported applications go, TenFourFox has always been behind mainline Firefox, VLC for PPC is stuck at 2.0.2, and I doubt StepMania 5 has a Mac/PPC build anymore. You can probably build some of that stuff yourself, but the build tools are so outdated nowadays that even rolling your own build can be a little rough.
I have an iMac G4 sitting on my desk that I pretty much only used it for compiling the Mac/PPC build of
MegaZeux and for playing Diablo 2. Considering Blizzard released a Mac/x86 build of Diablo 2 this year, it now only has one real purpose.