Summary:

  statmbx allows you to see what unread messages you have in your
  mailbox without opening an email client.  It is written in C and
  runs as quickly as is possible without indexing.  Compatible with
  mutt, pine, and any other mail program storing messages in the mbox
  format: http://www.jwz.org/doc/content-length.html

License:

  GPL v2 or later.  See the included file COPYING.

Usage:

  $ cat ~/.statmbx
  [events] mail/events
  [main] .mail
  [folder_with_no_new_messages] mail/somembox
  #[folder_not_to_check_right_now] mail/someothermbox
  $ statmbx

  [events]
    Jane Doe                     Re: [FOOLIST] Movies this evening?
    Ignatz Miller                [BARLIST] Gaming this Sunday!

  [main]
    Mike Smith                   long list, poorly organized
  $ statmbx --only-names
  [events] [main]

To get rid of the square brackets, don't include them in ~/.statmbx.
I just like the way they look.

If you don't want headers to show bold, perhaps because your terminal
doesn't do escape sequences, disable that by changing STATMBX_DO_BOLD
from 1 to 0 in statmbx.c and recompiling.

If this runs too slowly for you, you probably have too many messages
in the folders you want to check.  You could either switch to
something that uses indexing and caching, or you could move old
messages to some archive folder that you can then not check for new
mail.  I do the latter.
  
Installation:

  $ gcc -o statmbx statmbx.c
  $ cp statmbx somewhere-on-your-path