CNET's compiled a list of Facebook apps for techies. A couple of them are just geeky time wasters (retro arcade games and the official 100 question geek test) but there's a couple of interesting ones there including tools for embedding your blog into your Facebook profile, and a "text me" application.
I was also pleased to learn you could embed the xkcd web comic into your Facebook page. :)
Facebook apps for techies [CNET]