Emacs-IDE is an Integrated Development Environment (IDE) interface for Emacs with code browsing, compilation, debugging, etc. It aims to provide a user-friendly interface with dedicated windows (source, menu, and output) and convenient shortcuts. It is suitable for almost all languages (as long as they are supported by Ctags). Cscope provides an additional browsing facility for C/C++ files.
Geiser is a generic Emacs/Scheme interaction mode, featuring an enhanced REPL and a set of minor modes that improve Emacs' basic major mode for Scheme. The main features provided are evaluation of forms in the namespace of the current module, macro expansion, loading of files and modules, namespace-aware identifier completion, automatic documentation, jumping to the definition of an identifier, access to documentation, listings of identifiers exported by a given module, and rudimentary support for debugging. Geiser supports Guile and Racket.
AutOrg aims to be a text editor, personal organizer, local wiki, secure agenda, and everything in between. For the user interface, it gets its inspiration from text-based minimalism, still providing advanced features for publishing, encrypting, and sharing information. It uses Emacs Org-mode and other extensions, and its main binary distribution runs on a Mac OS X desktop.
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.