One of my favorite flickr games is fastr. Fastr tests your tag guessing skills by displaying photos from a particlar tag one at a time. You play against all the other users on the site and the first to guess the tag gets the most points. I love it because it is simple and it brings out the competitive side in me because I know I'm playing against other people. Fastr was one of the first flickr games to come out and its truly an oldy but goody. Check it out!