URGENT: Whole DB down ("no space left on device")

I am getting this message when I start the DB:

psql: FATAL: could not access status of transaction 0
DETAIL: Could not write to file "pg_subtrans/01F8" at offset 221184:
No space left on device.

What is this about and how do I solve this? A "df -h" on my system shows this:

Filesystem Type Size Used Avail Use% Mounted on
/dev/sda5 ext3 9.9G 2.5G 6.9G 27% /
/dev/sda1 ext3 99M 17M 78M 18% /boot
none tmpfs 2.0G 0 2.0G 0% /dev/shm
/dev/sda7 ext3 197G 17G 171G 9% /home
/dev/sda8 ext3 1012M 34M 927M 4% /tmp
/dev/sda3 ext3 9.9G 4.4G 5.0G 47% /usr
/dev/sda2 ext3 9.9G 9.5G 0 100% /var
/tmp none 1012M 34M 927M 4% /var/tmp

Please help!