I've found a fun new site. Project Euler is a huge set of math+programming puzzles of increasing difficulty, starting out pretty easy (solved by tens of thousands of participants) and going up to puzzles that haven't been solved yet.
They're quite fun. Each one touches some aspect of mathematics, and requires some programming smarts to get it working. But don't worry if you're not cluey on maths. They start out fairly simple, and you can google for information about the math behind the puzzles if you need more.
Each puzzle has a specific, numerical answer. You submit the answer to prove you've solved the puzzle, and the site keeps track of how many you've done. It also gives you access to a forum for each puzzle - where others share their code or insights into how to make the program more efficient. Most puzzles also come with a short pdf to explain the maths behind it - generally showing you how to analyse the problem or prove the formulae behind the most efficient algorithm.
Each puzzle has been constructed so that the correct program/solution is expected to run in under a minute... and for many of the puzzles - that means you'll have to rethink how your program works to get in under that time.
Anyway, I think it's an interesting challenge to play with. Here's my score so far: