Quote:
/var/spool/mail - where you mail lives when it gets to the system
/var/spool/mqueue - where outgoing mail is queued
/var/log - default spot for log files
/var/tmp - some things like to "swap" here
/tmp - the real spot for swap
There's some other "good" spots, I'm sure... though most stuff should normally end up in /var (that's the partition where "variable" files are supposed to live, normally). Mind you, if "/" files up, there's a good chance that your system won't reboot (if it's done by "root," it'll fill up the entire disk and create serious problems - normal (non-root) users won't be able to completely fill a disk, but they can give it enough to cause serious problems, partcularly if they're somewhat knowledgeable).