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.
Darius Tools is a collection of miscellaneous scripts which are useful for a variety of purposes. They are all designed to be faster and easier to use than the command line that would normally be required to accomplish the same task. For example, there are scripts for mass conversion of WAV files to Ogg files and Ogg files to WAV files, for converting LaTeX to PostScript and viewing the output in ggv, for creating image thumbnails, for converting from the Euro to another currency, for automated updating via FTP and for removing backups recursively. The Darius Tools are split among several packages according to different categories.