Pymacs is a powerful tool which, once started from Emacs, allows both-way communication between Emacs Lisp and Python. Pymacs in intended for using Python as an extension language for Emacs rather than the other way around, and this asymmetry is reflected in some design choices. Within Emacs Lisp code, one may load and use Python modules. Python functions may themselves use Emacs services and handle Emacs Lisp objects kept in Emacs Lisp space.
mu is a collection of utilities for maildirs, in particular for indexing and searching them. mu fills a database with information about your emails, which can then be queried using an easy query language. It can work stand-alone or be integrated with a mail client. It also provides an emacs-based email client ("mu4e") and bindings for Guile/Scheme.