Instant disposable email address and inbox: EmailOnDeck

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EmailonDeck provides an instant email address and inbox without registration or login that can be used to sign-up to services without submitting your official email or to correspond with others while remaining anonymous.

All received emails are automatically deleted after you have read them.

100% free. 

My comment: Simple, and immediate. Ad-supported. When in need it can be a lifesaver.

Try it out now: http://www.emailondeck.com/