That's useful for all those sites that want you to have a real e-mail address in order to sign up for a service, but which you might be worried about if they sell your address to other companies or spammers. However, there are two situations to watch out for:
- Sites where you're subscribing to an e-mail list. Obviously, you want a real address for that to go to.
- Sites that send a new password to the entered e-mail address if you forget your old one. If you're prone to forgetting your passwords, make sure you won't need this feature, or don't mind signing up all over again with a new ID.
Otherwise, Jetable looks pretty spiffy.
UPDATE: Fazal Majid noted another service that can avoid the second problem:
In a similar vein, try www.mailinator.com. If you have some uniform convention like "the email address I will use for acme.com will always be firstname.lastname@example.org", it's not hard to remember for lost passwords.
I've also added Fazal to my link list (on the right side of the page)—his low-intensity weblog is just that, and useful too.