SpyPig

Ever wondered if someone has read the email you've sent them?

That's where SpyPig comes in handy. Go to the website, pick the graphic you wish to use, input your addy and name, copy and paste the image into your message and when the mail is opened, SpyPig will notify you.

It won't work unless you use HTML mail or if the recepient has images blocked by their email, firewall or A/V program. (requirements and instructions)