I just learned about this one: ROSALIND is a really cool concept in learning bioinformatics. You are given problems of increasing difficulty to solve. Start with nucleotide counting (trivial) and end with genome assembly (not so trivial). To solve a problem, you download a sample data set, write your code and debug it. Once you think you are ready, you have a time limit to solve and provide an answer for the actual problem dataset. If you mess up, there is a timed new dataset to download. This thing is coder-addictive. Currently in Beta, but a lot of fun and seems stable.
Via Marthèlize Tredoux