Well I missed this – but apparently DP Review has launched a photographic challenges site.
Although still in beta, the site lists various mini-challenges for people to try and then vote on to highlight the best shots. No prizes as far as I can see (other than the adoration of your peers) but a nicely designed site. I particularly like the fact that the camera and lens used for each shot are not shown during voting. It will be interesting to see if certain equipment performs better than others.