Euler is a project mainly written in C++, it's free.
All my eulerproject.com solutions
Contains all my projecteuler.net solutions.
Update Sep 08 2011
In this project I'm solving the problems from projecteuler.net. The problems from 1 - 67 have been moved to a directory. I'll be putting new solutions into their own folders from now on. Till now using C programming design style (i've used generic functions sometimes though); from now on I'll be trying out object oriented design.