DidTheyReadIt.com?
I saw a brief CNN story on this earlier this week. This New York Times story (registration required) gives more details on how the service works. I already hate it when people set the Outlook feature that asks me if it is OK to let them know that I've read their email. They'll know I've read their email when I respond, y'know!
DidTheyReadIt depends on what are known as Web bugs, graphic files so tiny that they cannot be seen that can be embedded in an e-mail message or downloaded from a Web site. In the case of DidThey- ReadIt, a Web bug is embedded in each e-mail message sent through the service. When the recipient opens that message, the Web bug is downloaded and the DidTheyReadIt server records the circumstances of that exchange. But some e-mail programs do not automatically download Web bugs, and many can be configured so that they do not.
Mr. Rampell likened his service to caller ID, which can be disabled by users. "But that doesn't mean I'm going to throw away my caller ID," he said.