Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.
The intended audience include students for whom the basic curriculum is not feeding their hunger to learn, adults whose background was not primarily mathematics but had an interest in things mathematical, and professionals who want to keep their problem solving and mathematics on the edge
You can get more information on their website. They are fun problems that can be solved in a variety of ways and don't even always need programming knowledge, some can be done with a bit of thinking (and knowing the relevant piece of information). They can be done in a variety of languages, and you can see how different people did a problem on the site after you get it right (you have to register and submit your answer).
Please do not post solutions or answers in this thread. If someone wants their code reviewed or help on something, put your code in pastebin and provide a link, so people who haven't done the problem still can get a chance to do it.
I started on these tonight and have done three of them so far.
I hope everyone enjoys.