If you mix a little yogurt with brownie mix in a microwave safe dish, it comes out pretty moist and taste darn good too.
Another great one is just using 1 can of diet soda mixed with one box of cake mix, (diet root beer with choc cake mix, diet 7-up with strawberry or funfetti cake mix, diet orange sunkist with orange cake mix, etc.) cooked in the oven. You don't need to use the oil, eggs or any other of the stuff they tell you add. I can't taste any difference from the original box directions recipe!
"Just Do It"