My Kids Eat Free website lists what restaurants offer free kids meals. It isn't a complete listing, but might be helpful when planning a vacation to an unfamiliar area.
Locally some of the free kids meals I know:
Buehlers - Mon & Sat
Taco Bell - Tues night
IHOP - free kids meals 4-10pm (I think this is still going)
Chick-A-Fil - Tues night
Denny's - free kids meals 4-10pm (I think this is still going)
McDonald's - any regular Happy Meal 1.99 on Wed night