I often run into a system where one of the file systems has ran out of disk space. Usually this is because some application is producing errors and writing massive log files. Sometimes users will fill up their home directory with random large files they created or downloaded. Either way in order to fix the problem you need to know which directory is filling up the disk.
WinDirStat will scan the drive you select (or all local drives if you want) and display a graphical representation of the largest files and directories on the system. The columns are also sortable by their sub fields. You can click on the large blocks displayed to see which file it represents and easily delete it if you like.
On a Linux system you can use a program called Du to locate the problem.
du -x / | sort -rn | less
What exactly does this command do?
Du summarizes disk usage recursively, the -x option tells it to skip directories on different file systems.
Sorts the numeric output in reverse (highest to lowest)
Displays the output one page at a time. I like using less instead of more since it allows you to go back instead of just forward.