EmailOracle is a nifty service that will tell you whether someone has read your e-mail. It will also remind you to follow-up in case you have not received a response within a certain number of days. EmailOracle is available in the form of a browser plug-in for Gmail.
How does EmailOracle actually track whether someone opened your message? EmailOracle places a hidden image into every email you send. Once the e-mail is opened by the recipient, a notification is sent to EmailOracle’s servers, which will inform you whether it was viewed and how many times.
The service is available for free if you want to track only 20 emails per month. If you want to track a higher number, there are multiple plans available. The Basic plan is $9.95, the Deluxe plan is $19.95, and the Pro plan is $99.95 per month. The Pro account can track 10,000 e-mails.
Below is a video demo of the service in action: