I've long been interested in astronomy - I majored in it in college. So I just emailed a professor out of the blue. I looked for one whose personal page said a lot about "astronomy software." We met for coffee, and it went from there.

I think it's pretty easy to find tons and tons of scientific groups that are absolutely dying to find more professional programmers - at least in physics and astronomy, but probably in lots of disciplines. Feel free to email me if you'd like to talk more.