Email Oracle, a nice service and extension for Google Mail (Gmail) to track your emails. It's a great way to see if your recipient opens your email. Unfortunately the service is shutting down. So I've found an alternative. More after the break.The alternative site is called http://email-tracking-with-google-analytics.com and is not as comfortable as Email Oracle, but for the advanced user it's easy to use. You need to have a Google Analytics Account and a site where you use this Account (normally your self hosted website). No special setup is actually needed, you just have to insert the site url and the parameters for the tracking. After that you get a image url to insert in your email. This iamge is actually a transparent gif, so it is even better than the free Email Oracle Service.
The tracked email will show up in your Google Analytics report - goal reached.
This works with all email clients. For Gmail though you have to activate a Google Labs feature to insert images in the message body. Just click on the green "Google Labs" icon on top of your Gmail Window and activate the "Inserting Images" feature.
On the site you will also find some tutorials and a complete description on how the service works. So enjoy tracking.