I've been using Daily Caption since it started up. It's funny and competitive at the same time, and after a couple of days it can become addictive. Nice way to spend a few spare minutes at work.
Caption contests are one of those games that everyone and their mom enjoyed playing in forums. The concept is that someone would post some random photo and the community would add creative captions to that photo. My goal for DailyCaption was to take this idea and give it a nice "web 2.0" spin (sorry for the buzzword). On DailyCaption, Flickr photos are generated by community tags, participants can rate captions, and they can earn points to encourage bragging rights.