Metapost (main page, nice tutorial) is pretty cool.
In doing some homework for a class that’s heavy on mathematical notation, I had to get beyond the world of the word processor, so I decided to have another look at TeX. Good stuff.
I wanted to embed vector images. Additionally, I was biased toward PDF output rather than PS. That all led me to MetaPost.
Overall, it’s been a pretty big learning curve, but I’m having fun with TeX, MetaPost, and the general world of freely-available typesetting software.