Here is another programming challenge site. This one is specifically geared towards mathematical problems. You can use any programming language you like.
The problems are rated according to how many people have already solved a particular problem. The more people that have solved a problem, the less points that problem is worth. The scores are calculated dynamically, so your overall score can change depending on what the problems are currently worth.
I’m currently using Ruby for these problems. I still prefer Python, but Ruby does have some nice features.