I've been over to Jetable.org who offer short term disposable and re-directable e-mail addresses. The idea is you use jetable.org to create an email address at their site. Mail sent to this addres is then forewarded on to you. If you are posting in a newsgroup, for example, or signing up for a site whose privacy you are unclear about, you can just create a mail address and use it for a day or a week, or even a month at which point mail sent there just bounces and you do not see it. I've just created a one-month email address: firstname.lastname@example.org and email to this address is forwarded to my primary email account. This address should last until end-Sept 2005 - when it expires mail sent to the address just bounces.
At first sight, this seems a good idea - not letting your main email address get clogged up with spam. Of course, you do have to trust jetable.org not to sell your mail addresses to spammers or others!