Breakup Notifier, simply select friends who, in your opinion should be in your bed rather than someone else's, and you'll receive an email when (if?) they become single.
Speaking on behalf of romantics everywhere, the service is depressingly simple to use. Seriously, it's a 3-step process -- and to be honest, it kind of takes all of the effort (read: romance) out of Facebook-stalking your crushes. Anyone who has successfully chatted up a boy or girl online will know that timing is by far the most important factor, and with Breakup Notifier in your arsenal, rebound hook-ups are an easy, foregone conclusion.
Breakup Notifier is free for now -- but considering Facebook historically bans 'who defriended me?' apps, it might not be around long enough to make the developer any money. Better strike while the iron's hot, eh?