MSGTAG alerts you when your friends read email that you’ve sent them. It’s better than the standard Outlook Express receipt feature because it doesn’t activate pop-up windows on the recipient’s screen etc. And there’s a free version!

‘When your email is viewed at its destination, MSGTAG lets you know. You receive notification from MSGTAG telling you that your message has been opened. The notification also details when the message was sent, when it was viewed, and what the delay has been in between.’