I'm using Ubuntu 11.04. Today I suddenly started to get "low disk space" warnings. When I checked, I saw that free space on my Ubuntu partition is decreased like 2 or 3 gigabytes. I didn't install anything recently and I don't keep my files in my Ubuntu partition.
What could be the reason for this?
I found this to be a problem for me caused by log files building up.
logrotate is run as part of cron.daily which by default runs at 6:25 which is quite possible that your machine is never on at this time.
So I would start by checking /var/log disk usage.
If it is that, you can fix it by editing /etc/crontab and changing it to run cron.daily at 9:25 instead (or a time you know your machine is often on).
There is an application called "Disk Usage Analyser" installed by default. Use that on the whole filesystem.
anacron to run cron scripts.
It could be a number of things (internet caches, caches for other files, a runaway process storing data, etc.).
You can install Baobab (if it's not already installed) and use it to find out where most of your disk space is being used up.
Most likely, a buggy program started spamming error messages to stdout/stderr. These get logged to ~/.xsession-errors and ~/.xsession-errors.old, which can quickly fill several gigabytes of space Bug 60448.
A temporary fix is to truncate the file, by running > ~/.xsession-errors and > ~/.xsession-errors.old. (Deleting won't work, since the file is kept open.)
See also How do I prevent .xsession-errors from eating disk space?.
Press ctrl + h to show all your hidden folders. Look for .Trash1000. If such a folder exists, clear the files in it to get some space. You can also try clearing your trash and ~/.local/share/trash
