Zynga's CityVille is an unstoppable train that is just exploding out in all directions. It's the largest Facebook application ever and has only been around for just over a month after launching in December 2010.
Zynga have added 20 more levels to CityVille bringing the game up to a maximum level of 80. Currently I'm Level 31 (yes the game is addictive, I'm playing it because I told my mate that I'd beat him, haha). You might think this means nothing, but it's rumored that Zynga are paying Facebook $30 million per month for their 30% cut.