I'm looking for a fun math website with games/activities for my 3-year-old who is really (really, really, really) into numbers and basic math. He's good with the computer and can use it independently - but a simple interface would probably be better than one that is clutter or complicated. He knows numbers into the 1000s, and can do basic addition and subtraction.

I know there has got a be a website out there that he would just love. Or software would work too (but it's gotta be mac compatible) Any ideas?