Quotebook is a Facebook app that I wrote after being fed up with the lack of social interaction in the provided "quotes" section. I wanted to be able to look through my friends quotes, see quotes they'd put up by me, and send my favorite quotes to my friends - basically treat quotes like they were photos. After hacking around trying to make an extremely hackish version involving Greasemonkey and placeholder photos (don't ask...it was ugly), I discovered the Facebook platform.
I then proceeded to spend the entire weekend (the Friday through Sunday before finals, I might add) learning PHP and the Facebook platform to hack together what would become Quotebook, in June 2007. Now, nearly two years later, Quotebook has over 30,000 users, and I am continually working to improve it. It now allows you to add quotes just like photos, tag your friends in them, comment on them, and various other social-type things.
I have been pleasantly surprised by its popularity - I realized it had actually caught on when my roommate sophomore year (a few months after I created it) had friends back home that were using it, entirely unconnected to me or my friend group. This popularity has also resulted in investing far too many hours in making it work better, faster, and friendlier, a quest that never ends, to the point that most of my friends know that the default answer to "So, what have you been working on" is inevitably simply "QuoteBook".
That said, go ahead and check it out, and if you have any suggestions or questions, feel free to let me know.