An excerpt from the nmh man page (edited by YT). nmh is the current incarnation of the original RAND MH Mail.

mh - the true UNIX mail system

mh is the name of a powerful message handling system. Rather than being a single comprehensive program, mh consists of a collection of fairly simple single-purpose programs to send, retrieve, save, and manipulate messages.

Unlike most mail clients in UNIX, mh is not a closed system which must be explicitly run, then exited when you wish to return to the shell. You may freely intersperse mh commands with other shell commands, allowing you to read and answer your mail while you have (for example) a compilation running, or search for a file or run programs as needed to find the answer to someone's question before answering their mail.

Some of the various trinkets I use along with MH.

TCSH aliases I use to make life just a little bit easier.
Extended version of Jerry Peek's fols to emulate ls.
This is the second incarnation of a program I wrote to quickly send email from the command prompt. It's not been fully vetted and made portable. configuration file
Generates text-based histograms of folder contents by sender using perl's Text::BarGraph
Extended version of Jerry Peek's recomp.
My rewrite of the SIPB script spam-inc to accomodate filtering of messages tagged as SPAM by SpamAssassin.
A trés simple wrapper for send to append the "standard" ~/.signature to outgoing messages.
My MH profile. The defaults for dist are probably the most non-standard thing there, but quite useful.
output a range of values; one per line. This is particularly useful for handling MH messages with standard UNIX tools, granted you could do some similar things with pick.
My simply modified scanrc to include year data.
visual pick, mark a message sequence by eye. If you want to try it out without installing the dependencies I've built a binary (Perl 5.8.0, i686 Linux). More information; man page and screen shot.