Furtivue : a furtive view over your messages. (www.furtivue.com)
Furtivue is a web service that allows you to send temporary (self-destructing) emails, as in, you can control how long the message is going to stay with the recipient.
When they open your Furtivue message, it appears for a specified number of seconds, then self-destroys. The duration will depends upon the length of the message (Or you can choose how long you want the message to be displayed).
How it works:
User A writes a message to user B, a link is sent to B. When B clicks on the link, the link is de-activated (therefore can't be used thereafter), the message is shown from a Silverlight canvas and disappear after the predefined time is over.
This could be used for sending confidential/secret information (such as passwords), or any other data you don't want the receiver to keep permanently in their e-mail box : for some reason, you may want to express yourself without leaving a permanent record.
You can try it here or download/explore the source code from here.
Image source: allpstrophies.com