Can't Eject A Volume Because it's in Use? Use lsof.

macosxhints has a really handy bit of terminal magic to find out just what's in use when you can't eject a volume.

Low Disk Space Script

I haven't tried this but it just seems like it could be handy somehow.

Low Disk Space Script "Low Disk Space Script is a simple AppleScript that tests two things:
That there are no hard drives missing
That there is a certain (pre-defined) amount of available space.
If one of these tests fail, it then automatically sends an email (to one or more pre-defined addresses) to inform you of the relevant problem."