You go to a web site, you want to download something and you're required to give an email address, possibly to confirm you're a real person.  In most cases, I create an alternate SMTP address on my mailbox, acknowledge their message and then delete the address from my mailbox.  As an alternative, this site enables you to create any email address and receive messages which can be viewed through their site.  Be warned, EVERYONE can access this email so I would only use it in cases when the received mail is meaningless.

http://www.mailinator.com/index.jsp