Melt Mail provides free, disposable email forwarding
Many require you return to their site to check for messages, but Melt Mail lets you work with any real email address.
Melt Mail works with your existing email, setting up a temporary forward @meltmail.com. You've got four time periods to choose from - three, six, 12, or 24 hours. After that, your address expires and forwards from it to your real account cease.
Once your address has been created, you can choose to pop up a small reminder window that displays your address and an expiration countdown.
One source of concern I have with the service is the lack of a privacy statement. If you're going to use Melt Mail (or a service like this), check for one before signing up. Who's to say your real address won't end up on someone's mailing list.
Melt Mail is a slick service, I'll just stick to using it with my "registration" gMail account.