Review on EmailAnalytics by Pedro Movie

It's free, which I love! The ability to view your open rates (among other things), as well as senders/recipients of emails you have sent in the past are both very useful. Also, it has some great integrations with Mailchimp - so if there was anything that we needed help with, they were able to point us in the right direction. If you're looking at using this tool, make sure you use their integration services. We've used them before when our mail provider went down, and they got back up almost immediately after contacting them.