A fast, global-search and tag-based email indexing system using Xapian for the underlying indexing and searching.
Notmuch is a library usable from C or C++, with a command-line interface program, interfaces to use Emacs or Vim as mail clients, and bindings for Python, Ruby, and GO.
And now, also for Tcl! - see http://chiselapp.com/user/emj/repository/tclnotmuch/home .