Project Euler is a collection of challenging programming problems. There are over 400 problems, as of writing this. That is enough to keep anyone busy for at least a year! One of the many great things about Project Euler is that the problems are language agnostic. Whether you use Ruby, F#, C, etc. it does not matter. What matters is your ability to approach the problems programmatically and solve them.
For example, here is Problem 3:
The prime factors of 13195 are 5, 7, 13 and 29.
What is the largest prime factor of the number 600851475143?
If that is not an exciting challenge, then I do not know what is.
I have been slowly progressing through each problem. Some take me a few hours and others a few days. It has been great to revisit a lot of math that I have not used in years. A lot my time programming lately has been for business purposes and not for learning purposes - this is a huge problem. Working on Project Euler is one of my efforts to do more fun programming. More time needs to be spent programming for fun and learning.
I recommend anyone who is interested in program try and solve Project Euler problems with their favorite language or a new one. There is not much better than the rush of solving a problem.