MH-E is the Emacs interface to the MH mail system. It offers all the functionality of MH, the visual orientation and simplicity of use of a GUI, and full integration with Emacs and XEmacs, including thorough configuration and online help.
I've been using mh-e for... a really long time, and it's completely indispensable for my day-to-day work. MH (nmh) is still the best e-mail client I've ever used (and I've tried a lot of them), and mh-e is the best interface (because, of course, Emacs for the win).
Since I run this project, you'd be concerned if I gave it a thumbs-down review! :-)
MH-E is a super way to run the MH system -- it does not disturb command line usage, and it is totally integrated with Emacs as well