Favorite Open Source Software?

There is much in my queue to post, as usual, but tonight belongs to networking at the CUE conference… except for a few quick questions.

I am still finishing up my “Free as in Freedom” presentation – an introduction to open source software in education. I’ll begin with a quick demo of course, followed by an introduction to the history and philosophy of open source software. Then I’ll demo specific applications. Of course I’m covering all the big names: Firefox, GAIM, GIMP, and Open Office. I’ll also cover some open source web platforms like WordPress.

I want to include lots of the little gems, too, and am reviewing my own harddrive and asking my colleagues what else they are using. So I thought I’d pass that question on here… for a mention in my Friday morning presentation… what’s your favorite opens source software (that I haven’t already mentioned)?