The device is full. Note that by default, on ext3 partitions, normal users cannot use up 100% of the disk space. 5% is reserved for root, so root has space to bzip2 files, move things around etc. This 5% margin also lowers the amount of fragmentation that occurs on a filesystem. This may mean that even though your partition may not be totally full yet, programs will complaing about being out of space.
Depending on your tools, they may say that "100%" of the disk is full at this 5% margin (and therefore you can have up to 105% of the disk being used), or they may only say 95%.
These settings can be changed with tune2fs(8).
Linux offers a convenience for testing how code reacts to a full device in the form of /dev/full, a device which returns ENOSPC for any attempt to write to it.
