My latest obsession: whatsbetter?com.
Kind-of along the lines of the AmIHorOrNot? phenomenon, there is a huge database of pictures, which are randomly selected and presented to visitors 2 at a time, who then get to choose between them and determine which is "better" (by whatever subjective standard they care to use). Over time, items get assigned a "ranking," determined by the sum of their votes (either +1 or -1) to date.
Users can also upload their pictures, which get added to the database. This is where it gets fun. You sometimes end up with some truly bizarre pairings. For example, right now, I am being called on to choose between "strange french guys" and "king-fu kitty." The titles (also user-submitted) to the various pictures are often quite humorous as well.
Like I said, hours of mindless fun!
(Unfortunately, this server has a bit of a hard time coping with the recent increase in popularity at times...)