Via Joel On Software, an excellent Web utility: the Mailinator.

The concept is very simple: if you need a disposable e-mail address right now, just send a message to any random address @mailinator.com. The address is created instantly and remains accessible for a few hours. Very nice for sites that require an e-mail address (often to just to sell it later). Obviously, as the site states, you shouldn’t use the service for messages with sensitive data.