While the preponderance of new apps have been developed for use within the Facebook site itself, some developers are also building Facebook apps for the desktop. For example, FriendVis lets you visualize your Facebook friends network, while fbquick and FMenu give you desktop notification of feed items for Windows and Mac, respectively.
One app that I actually use is Friend Photos. It’s a screensaver for Mac OS X that pulls in photos of my Facebook friends for use in a screensaver slideshow. While this may not be a work-safe app (depending on your friends :)), it’s a fun way to see photos of friends that you might otherwise have missed. (Windows users: check out FriendSaver.)