Project Euler
This is a wonderful website offering a long list of interesting and challenging math and computer science problems.
There are many different kinds of problems to solve: some are purely mathematical in nature, and some would be considered more like pure computer science.
Some are easy (“What is the 1001st prime number?“), some seem moderately challenging (“What is the sum of the digits of 
The problems are freely available, but it looks like you have to register to submit answers.
3 Comments
Leandra Stewart · March 29, 2011 at 3:02 pm
How many have you solved?
This is so reminiscent of senior year math team competition questions
MrHonner · March 30, 2011 at 9:10 pm
I haven’t solved enough to brag about. 🙂
And yes, very math team-y. Some of the comp sci questions in there are really interesting and worthy of investigation.
Leandra Stewart · March 31, 2011 at 4:14 pm
I’ve found excel to be a great resource in figuring out the patterns of some of the problems.
I started it on the 29th and I’ve done 4, with three others partially solved.
Quite an interesting thing to do in my spare time