Here is where I will dump challenges I complete from Reddit's "Daily Programmer" subreddit. http://www.reddit.com/r/dailyprogrammer - It's great fun.
Daily programmer is a forum where the moderators release three challenges per week, on Monday, Wednesday, and Friday. The challenge difficulty scales with each release (i.e. Mon is easy, Wed is medium, and Fri is hard).
With each release the challenge is described and a program specification is given.
Beyond that it is up to the individual to choose the right tools for the job, create a program, and submit the code. The top submissions are awarded "medals" and there are weekly discussions on various programming topics, check it out!