I found out about Hooda Math when the creator of the site posted a guest entry on Emanuele Feronato's Blog. Michael Edlavitch is a math teacher who made some educational math games while he was between teaching jobs. He made a website to host his games, and then decided to host other math-related games as well. Now he owns a successful niche Flash game site with 10,000 visitors per day! What interests me the most about this story is that Michael's games are based on the classic Apple II game "Number Munchers". Number Munchers was was one of my favorite games for the Apple II when I was a kid. I played it outside of school way more than I did inside. I was motivated by wanting to see how each of the different types of bad guys (called "Troggles", if I recall correctly) behaved. I also loved watching the humorous animated cutscenes you would earn after every few levels. Wanting to see more of those kept me playing for longer than I probably would have. Depending on your age, you may have played the Mac version of the game instead.
Zero Sum is another math-based game that ends up being pretty fun. It's a standard Match-3 style game like Bejeweled. The difference is that instead of swapping two tiles, the first tile you click gets added to the second tile, and the empty space is replaced with the tile waiting at the front of a queue of tiles on the right side of the screen. The tiles only contain the ones place of a sum, so in this game 9+1 = 0, 5+6 = 1, etc. The "zero sum" mechanic comes in when you add up numbers to equal 10, because a 0 tile is a wild card that can match with any two other tiles.
Math is the driving force behind most arcade-style games. The process of programming a game with a ship that can rotate and fire in a 360 degree arc, with missiles that accelerate and explode when they reach a target coordinate, expanding into a radius derived from the missiles power level, damaging nearby ships, and pushing them at the correct angle with a force that drops off exponentially from the distance of the center of the explosion, has given me more practice with trigonometry than all of the math homework I've ever done, and it certainly was more relevant and interesting to me.