A clever pseudo-hack that allows a web site to determine if I'm logged into facebook / gmail / digg / possibly other sites based on various urls which are only accessible to logged in users. Obviously smashed by noscript... :-)
Online coaching, feedback, performance reviews, that kind of jazz, for teams or businesses based around facebook or something of that nature.
Interesting article about how effective facebook ads can be with some optimisation. Cost per clicks between a few pence and .3p per click. Food for thought. No specific advice or tips in the article.
Pimp your facebook fan page for $40 plus.
Sell on facebook and other places. They handle everything for 7% plus 3% + 30c for card processing, but they'll take cards for you, no merchant account required, through elavon.
AdBlockPlus filters to block Facebook ads. :)
A pidgin plugin for Facebook Chat, it adds Facebook as a protocol to pidgin accounts.
Ping.fm allows you to update a bunch of social networks in one go. Post to ping.fm and have it passed on to Twitter, Facebook, plurk, myspace, linkedin, plaxo, bebo, etc, etc.
A planned, open-source hopsitality exchange system for Facebook.
Facebook developer portal, documentation, resources, and so on.
An article from Sitepoint.com on how to develop PHP apps for Facebook