Mailgate for CVSTrac
As the name implies, this takes care of sending outgoing "Change Notifications" as well as, if you want, processing incoming e-mails from known users for creating new tickets and appending remarks to existing tickets (if the subject has a valid "[project #ticket]" tag in it).
It works great for me, but I'm considering it beta quality until it gets more mileage. Just read README.mailgate in the tarball for installation instructions.
Latest release: Mailgate 0.4beta