https://marco.org/2010/07/22/cow-clicker-you-get-a-cow-you-can-click-on
You get a cow. You can click on it. In six hours, you can click it again. Clicking earns you clicks. You can buy custom “premium” cows through micropayments (the Cow Clicker currency is called “mooney”), and you can buy your way out of the time delay by spending it. You can publish feed stories about clicking your cow, and you can click friends’ cow clicks in their feed stories. Cow Clicker is Facebook games distilled to their essence.
(via inky again — I guess he’s writing my blog while I’m busy with cache serializers this week)
This is the first Facebook app I’ve ever been tempted to install. And Ian Bogost’s rationale in creating this is worth a read.