There are a few services out there that allow you to sign up for free disposable e-mail for those times you want to set up registrations and have forwards set up, then expire at a later time or date. Two that I have recently read about are MeltMail.com and Spambox.us.
Melt Mail doesn't require that you return to the site to check your messages, since it forwards everything to your chosen "real" e-mail address for a period of time. No messages are stored on the site and your e-mail is deleted and forgotten once the time expires. Setting up a temporary forward to an address @meltmail.com allows you four time periods for the forward to follow: 3, 6, 12, or 24 hours. Short-term but useful, you can also choose to have a popup reminder window which displays the address and a countdown for when the service will cease.
Spambox.us offers a similar service but there is no popup reminder and it will forward your mail for up to one full year. Here are some details from the site:
We create for you a temporary e-mail address that will expire in the time you chose, all the mails directed to this e-mail will be transparently forwarded to your real e-mail.
For example, you need to register on a forum that requires e-mail verification, so you generate a spambox e-mail with an expiry of one hour. The generated e-mail will expire in an hour, way enough for the Forum to send you the verification code.
Take a look at these sites and if you are in need of a service like this, let us know which you chose, and why.