What OS is it running? 10.4 has a lot more software options than older versions of Mac OS X.
This is a good list of web browsers that might work on your TiBook

. Bean and AbiWord are pretty good text editors, and MuseScore is a good music notation editor. I'm guessing you've heard of MacTubes. zen.state has posted a G4-optimized version of mplayer that plays .mp4 files much better than QuickTime. Also, VLC is a pretty good media player.
ThemePark is a good app for changing the interface