Find out about who opened your emails.

I have to admit that I haven't tried this one myself, but I've been told it's good and it's also free, which is never a bad thing. If you use it, let us know what it's like.

http://www.spypig.com

(edit) Site is dead. Try one of the others listed below.

following your suggestion about spypig, i have proved it, its good but very limited on time, anyway, i have found another one which i have proved myself, its also totally free and allows you to send until 20 emails per day. it works similar to readnotify, but its simpler.

http://whoreadme.com/

try it.

And i have another one: getnotify.com. Complete free, allow multiple accounts and can be used for unlimited number of emails. I used it myself
This one seems to have one single problem: sometime the location for IP is wrong and it´s better if you double check the IP location with another tool.