Monolingual is free software for the Mac that helps eliminate extra languages included in Mac OS (like Bulgarian and Portuguese) as well as binaries optimized for systems other than your own. I got the idea from this MacWorld article that also has some other tips on slimming down your OS X computer.
My G4 saved about 500 megs, my Intel Mac saved about 3 gigs.
Side note: despite all this, I'm very interested in moving everything over to Linux. If something happens to my Mac Laptop, I don't have the money to buy another one. Still, it helps to maintain what you've got.