There is a tool called EmailOracle that allows you to track if your email is opened, read and so. It inserts a pixel in the email which is hosted in their server. Then if it is accessed they can build stats on that.
The cool thing is that they have a Gmail extension ;-)
The service is free but if you track it inserts a banner. If you pay, it is clean. If you don’t register it is impossible to see how much you should pay for a full service, which is very nasty. Just for this I will not even sign in to try.
A work around could be to insert a signature (jpg), hosted in your google docs or dropbox or anywhere, and track it with google analytics. For free, forever. Not as cool but it works.