After creating a disposable e-mail address, the service gives you a unique (https) link where you can even change the address settings. There is also a Firefox extension for this service (available at Mozilla).
Spam.la - www.spam.la
This online service should be used just for non important throw-away site registrations as all the e-mails sent to the address you chose will be publicly readable on spam.la homepage. Quite a lot of people are using the service, so you can easily filter the listing with your own disposable address to see the actual mail you received.
This were just some of the unique disposable e-mail services, but I tried to cover just those which don't need user registration. If you are interested in other alternatives - Google is your friend.