I guess it depends on what you are into. There is a lot of really cool audio software out there that doesn't exist on the PC.
SuperCollider - geekout audio language - you can get on win/linux but it started on mac and has an awesome gui for it.
soundhack - sound file processor
cycling 74's M - this is an old school midi app updated for osx... awesome
JackOSX - let's you route audio all over the place... something swiped from linux.
Digital Performer - fantastic DAW
The dev tools are pretty sweet if you are into that sort of thing. You can download from Apple for free.
Oh yeah, if you are into unix stuff, Terminal.app
look in your Applications/Utilities folder.
There's all sorts of wonderful stuff out there for mac. I've been using it for my primary OS for a lot of years so the novelty of finding new apps has somewhat faded. I'm pretty set in my ways. I guess once I upgrade to a new macbook I'll probably start looking around.
btw - best place to look for apps is versiontracker.com.
If possible, I'd suggest a RAM upgrade. Tiger should run decent on a 600mhz machine (g3?) with enough ram. You should be able to buy a chip for next to nothing. At least 512 will give you a great boost.
If you've got any questions, feel free to PM me.
