Oh c'mon, that's what Folding @ Home is for. If you can snag an inexpesive but fast computer, you can use it to cure diseases and stuff.

The old, interesting ones aren't fast. This one, for example, would be about 200MHz.

Not blazing fast, but still enough to do a little work.

Yes, there are a lot of things it could do. Sadly, there are only so many web servers (or whatever) that one person needs. You can start hosting things for other people, but then you rapidly run into electrical and thermal limits. Trust me, my computer room uses three circuits and I have to keep the windows open until it's well below freezing outside, and that's with only seven computers running continuously.