ProjectEuler is a website dedicated to a series of computational problems intended to be solved with computer programs. It includes over 500 problems (July 2015).
HackerRank is a company that focuses on competitive programming challenges. Here is the ProjectEuler+ contest.
People from HackerRank have modified the original ProjectEuler challenges and turned them into harder versions by increasing input size, adding hidden test cases, and setting time limit for your programs. They have redesigned the first 100 challenges so far (July 2015).