chgrep searches the input files (or standard input if no files are named) for oldstring and changes them to newstring.
Searches a normal or gzip'ed mailbox for a given regular expression, and returns those emails that match it. Also supports searches constrained by date
gtkgrepmail is a gtk front-end to grepmail which allows you a GUI interface to search through your sent and saved mail.
finds threads in a bunch of mail messages and outputs them in various ways. It is rather sophisticated and can be controlled by various options
mairix is a tool for indexing and searching email messages stored in maildir folders.
a small utility that scans standard Unix mailboxes and displays messages matching a basic, extended, or Perl-compatible regular expression
a small perl utility which displays messages from a mailbox depending on a search pattern
mu is a set of command-line tools for Linux/Unix that enable you to quickly find the e-mails you are looking for, assuming that you store your e-mails in Maildirs