http://mailinator.com – no registration, no nothing. Use <something>@mailinator.com in any registration webform, and then go to mailinator.com and check for your registration mail just by entering your <something>
http://10minutemail.com – go there, click link and copy your new mail address. Use it and wait for registration mail. Your mailbox expires after 10 minutes unless you refresh.
inbox.com – regular webmail, but when you receive new mail it first goes to ‚verify‘ folder. Sender receives captcha test, and only after he solves it, you receive the e-mail. Test has to be solved only once for every sender.