Fastr is a great game that pits you against the other users at currently at the site. The game is images that are slowly displayed on the screen and the users much each guess at what the common tag that is shared by all of the pictures. Its a fun, extremely addicting game. Be warned: Don’t start playing this at work.
FlickrBabes isn’t much of a “service”, and its not exactly “Web 2.0″…but it is fun. These guys just crawl flickr and pick out the “babes” and post them on their site. Simple and effective. How can you not love it? They even make sure to tell the user that posted the original image by leaving their tag in the comments telling the user that this picture has been found by FlickrBabes. Nice touch.