Now here's a great idea: Register with the Takes All Types facebook application, tell them your location and blood type, and they'll contact you when your blood is needed.

TechCrunch notes that, despite 43,000 pints of blood donated each day by Canadians and Americans for 4.5 million patients, total donations aren't enough:

Historically, the Red Cross will make efforts to alert the public during a shortage. But there may be a better way -- leverage the social networks to get the word out. If shortages of a certain type of blood occur in a certain ZIP code, having a database of willing donors in that ZIP code to contact may be the most efficient way to solve the problem quickly.

This is even better than that Vampires vs. Werewolves game.