It is Spring and as every year it is time to do a bit of cleaning. Perhaps, you have recently upgraded your Ubuntu Linux from the release 6.10 (Edgy Eft) to the up-to-date 7.04 (Feisty Fawn). By doing so you might have some packages and residual files that are no longer needed, and by removing them you could free up lots of space on your hard disks.
To describe the various possibilities and reasons to do an Ubuntu Spring cleaning, I have started a wiki article. If you have any comments feel free to give them here, in reply to this post. I will try to take them into account.