twitter!

So useful! especially when there are people like Dan Cohen from the Center for History and New Media on it. I've already been sent links to useful sources (journals en español, for instance) that I probably would never find on my own.
History not being a typically collaborative field, the ability to fire off a query into the unknown and quickly receive back useful tweets is amazing.
(if you want to follow me, I have the amazingly creative username: http://www.twitter.com/katrinagulliver)