Good info so far. Just noticing that nobody brought up that having only 500MB of space free on a 40GB HD under any OS is typically bad. Mac OS handles this generally pretty well right up until you run out (at which point it can do really nasty things like not even boot, etc.)

You may find that you gain up to a couple GB after using something like OnyX. There are some log files which will run right up to the 2GB limit if they don't clear. In the latest versions of Tiger it's actually set up so that if your Mac is not awake in the middle of the night when the clearing scripts usually run, they will run the next time the computer is awake. Earlier systems need to do this manually every once in a while. OnyX and some other utilities have sprung up to take care of this.

That said, though, you really should bump off about 4GB at least to have 10% free. That would be better all around. (Some people suggest up to 20% free, but I think that's extreme in most cases.) Plus, after getting some free space, you might want to pick up something to optimize and defrag. Yes, there is a certain amount of defrag that happens automagically, but it doesn't cover everything, and a scenario like completely filling up the disk throws various things into disarray. It won't stop working without it, but it'll work more efficiently (faster) if you clean it up a bit.