At various times, I have:
taught/tutored SAT, GMAT, GRE
done contract work for Educational Testing Services (scoring and developing the TOEFL online - you need an MA in TESL for this, but there are other tests without these requirements)
taught an ESL course for a friend in Costa Rica
sold clothing on eBay
been a promotional model/spokesperson for various products
I stopped doing any side jobs during grad school, but I should start picking this up again now that I'm finished. I recently started a blog as an experiment (not sure if this will ever become profitable), I may try to pick up the work with ETS again, and I have an offer from one of my professors to teach a class in their program. I've also considered trying to get certified as a fitness instructor or taking tax prep classes and trying to do that seasonally.