Nomics was recommended to me by Loyal Blog Follower Joel. It's a math game where the only operations are addition & subtraction.
I got to level 26 and found it very interesting. The graphics could be a lot better. But the puzzles get pretty tough.
Sorry about not putting up images, which would vastly help in explaining the rules. I have not been able to access the link below. Maybe this game is only available in Europe? This is definitely the type of game that's actually pretty good but will never be popular.