Project Euler is a website where you can find a large collection (300+) of computational problems of varying difficulty. You can use them to exercise (and improve) your programming skills.
- Calculate the sum of all the primes below two million.
- Starting in the top left corner in a 20 by 20 grid, how many routes are there to the bottom right corner?
This project has been around for years, but I only just stumbled on it via another post.