Mailinator

SpamGourmet

You have to fill out a form, but you're worried that your email will harvested for spam. Enter "Disposable Email".

Mailinator is the easiest to use. Whenever you need a disposable email address, just make up any word, then follow it with "@mailinator.com". You don't need to register in advance. Go to mailinator.com and enter that word. You can then retrieve any email that has been sent to that address. It will expire automatically sometime in the future.

SpamGourmet is more sophisticated and requires you to register first with a real email and a user name. Once registered, you can make up words like mailinator and enter them into forms as "madeupword.username@spamgourmet.com". Emails sent to this address will be forwarded to your real email account until you cancel that fake email.

http://mailinator.com/

http://www.spamgourmet.com/