HOWTO Get a load of hard-disk space back

A handy tip -- if you use Thunderbird to get your email, 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.