There are a lot of vsites that provide you a facility to send anonymous emails to others.The concept uses the fact that the IP address of the sender is not passed in the message header so the recipient will never know who sent the email.

This service can be useful for the following reasons:
Catch a cheating spouse husband or wife.
Inform the tax office about tax cheaters.
Confess your love to somebody.
When your own email service doesn't work.
Report fraud to your boss or institution.
Find out if your friends are real friend.
Give warnings to people.
Inform the police about illegal activities.
Play an email joke with your friends.
If your private email is banned by the recipient.

Other than sending anonymous emails, this service will also come handy if you are sitting on a public terminal and want to quickly send yourself an email without logging into your own web account for security reasons.

Anonymailer is a great site for sending anonymous mail I found on  Technospot which provides facility for sending free anonymous emails plus it is also a free proxy site that can be used to reach to pages that have been blocked at your workplace.

 
Amit has also written a post on another free tool like this called Note2Email.Also my friend Shitu has given a long list of sites that offer such a facility.