I was editing on a computer that I don't normally use today and suddenly Mac OS X yells that Your Startup Disk is Almost Full, from out of no where.
Being on a machine I've never used before I had no way of knowing what had been done to it. Usually this stuff is caused by caches, render files or someone setting the capture folder to the wrong place, or a surprising amount of the time, multiple exported full-res versions of a project.
I looked around at macupdate and found an app called DiskWave which shows sizes of folders in a pretty simple to use format.
In this case it was Adobe's Cache folders in the user's Library folder, about 5 GBs worth. I trashed 'em without asking the client and went on working.
So if you run out of space this little free app is pretty handy.