I haven't used it yet, but I'm starting a campaign in maptools next week. I've tried some others and maptools looks like the best free options I've seen... and better than many for-pay, too.
You already know but just let me say once again MapTool, no question our group could not be happier with it.
And I say this as someone who bought and paid for four licenses of another, paid-for, seemingly-higher-quality online tabletop, MT is free, opensource (I think) and in active development and still comes out head and shoulders on top.
OpenRPG I've heard about but never got around to try.