For all those things you can’t join without giving your email address. Of if you have other reasons for sending an anonymous email. (But, let’s face it, all of those reasons are sinful.)
http://10minutemail.com/10MinuteMail/index.html
You go to the website, it gives you an email address. You use it to register for something or whatever. Then look again at the 10-minute email site and there’s the “confirmation email” in the inbox. Act on it. 10 minutes later, the email address goes away.
To anticipate, some people think services like this help spammers.