http://www.open-cs.net/ is an online judge site that features official solutions to all of its hosted problems. I hope this approach will eventually make more people interested in algorithmic challenges, as right now I think that most similar sites are too elitist: a lot of problems are too hard for someone without very specialized training to solve, and not everyone has access to such training.
Of course, for this to work it needs a full-fledged community to work with, so I'm trying to get people interested. I hope you like the site and consider participating.