Hmm, yep, looks like it really is full...
Here's something you can try:
find / -size +10000k
That'll find files anywhere that are larger than 10MB (see the Linux Cookbook
for other ideas). Maybe there's some enormous garbage-file somewhere taking up space. I suppose it might be a bunch of small files, too, but a few large files is probably more likely.
Also, look for any core files:
find / -name 'core*'
(that's quote, core, asterik, quote) Any of those floating around can be deleted immediately - they're just a dump of your RAM when a program crashes.