If Thunderbird's your main email application and you've got thousands of messages stored on your hard drive, they may be hogging more space than you think. Writer and T-bird user Cory Doctorow says:
Don't forget to occasionally run File -> Compact Folders. I did so yesterday and reclaimed nearly 20GB of hard drive space! Comparing my mail folder to my backup, I discovered that every single email that I'd "deleted" for over a year (by putting it in the Trash and then emptying it) was still lurking on my disk.
Twenty gigabytes is a whole lot of storage to get back from a simple folder compaction. Of course, to ID more space hogs, here's how to visualize your hard drive usage.