I occasionally do FreeBSD / security / cryptography / algorithms consulting; it doesn't make much money, but I find that it's worthwhile just for the sake of helping keep me aware of what's going on in the world around me (if I'm interested in something and not too busy, I'll normally offer to spend an hour or two listening to people and throwing out ideas for free).

It's not really about easing the bank drain, though -- I take consulting work when it comes up, but I don't chase after it.