In the mail:
< img src="http://mydomain.com/mypixel.gif" alt="">
On the servers command line
grep mypixel.gif path/to/access.log
Provided you use an apache server and the logfile is called access.log (should be the default). Look at the grep manpage for options and exact syntax, but it doesn't get much less complicated or more open source than that (it might be that your tracking efforts are thwarted by the Gmail image proxy, though).