I have squandered hours of my life (and certain friends, you know who you are) using one app, which has been terminated under the threat of a lawsuit: Scrabulous. I haven't tried the reinvented version of it yet, but according to this article, something like Scarbulous will be reinstated soon if it hasn't already.
The downside to Facebook releasing it's API for developers is that a lot of nonsense apps have been created. Several weeks ago, I decided to begin my personal ban of them, saving up for just the right moment to hit the "ignore all" button. I figure a nice round number like 100 is a good time.
Here we go!