Despite the funny name, excellent collection of ideas for adding game mechanics to a web application. A friend of mine succeeded in doing just that after reading this cracked.com article, but this website is far more complete. Particularly the Game Mechanics section, which lists no less than 33 mechanisms by which to make an application more game-like.
The act of inducing player behavior not by giving a reward, but by not instituting a punishment. Produces consistent level of activity, timed around the schedule.
Almost all startup products can benefit from these. This site is a gold mine.