Programming Support

ANTLR Mode
Major mode that provides various features for editing ANTLR grammar files.
http://antlr-mode.sourceforge.net/

Bf.el
A BrainFuck interpreter written in Emacs-Lisp. To demonstrate that the implementation is really working, a weird test-case is included: An interpreter written in BrainFuck compiled to Emacs-Lisp executing BrainFuck code.
http://www.emacswiki.org/cgi-bin/wiki/bf.el

CC Mode
The development homepage of the Mode for editing C/C++ and other languages with similar syntax.
http://cc-mode.sourceforge.net/

David Ponce's Emacs Page
Additions to JDE.
http://pagesperso-orange.fr/david.ponce/

ECB
A source code browser for Emacs. It is a global minor-mode which displays a couple of windows that can be used to browse directories, files and methods. It supports method parsing for Java, C, C++, Elisp. [Open source, GPL]
http://ecb.sourceforge.net/

Emacs ada-mode homepage
Mode for programming in ADA. Downloads and documentation.
http://stephe-leake.org/emacs/ada-mode/emacs-ada-mode.html

Emacs Development Environment Tools
CEDET is a collection of tools written with the end goal of creating an advanced development environment for Emacs users.
http://cedet.sourceforge.net/

Haskell Mode for Emacs
An Emacs mode currently supporting font locking, declaration scanning, documentation, indentation, hugs interaction.
http://www.haskell.org/haskell-mode/

Hugs Mode for Emacs
Provides fontification and cooperation with Hugs, by Chris Van Humbeeck.
http://www.haskell.org/libraries/hugs-mode.el

Ilisp
Integration of various Lisp implementations (mostly Common Lisp systems and various Scheme dialects) within Emacs.
http://sourceforge.net/projects/ilisp

Inform Mode
A major mode to edit programs written in the Inform programming language. Supports the usual program mode functions: automatic indentation, moving over expressions, comment and string filling, font locking, tags-file support, starting compiles and parsing errors.
http://www.rupert-lane.org/inform-mode/

Java Development Environment
A software package that turns Emacs into a comprehensive system for creating, editing, debugging, and documenting Java applications. Information about installing and using JDE, FAQ, package download and contributions.
http://jdee.sourceforge.net/

PCL-CVS
Emacs interface to CVS, the open source Concurrent Versions System.
http://ximbiot.com/cvs/cvshome/dev/addonpclcvs.html

Phillip Lord's Emacs Packages
Mostly additional packages for JDE.
http://homepages.cs.ncl.ac.uk/phillip.lord/emacs.html

PHP Mode
Sourceforge project page of a major mode for editing PHP source.
http://sourceforge.net/projects/php-mode/

psvn.el
Provides a subversion interface for Emacs. Information on the module and the source code are available.
http://www.xsteve.at/prg/vc_svn/

Python Mode
Python support for Emacs and XEmacs.
http://sourceforge.net/projects/python-mode/

SQL mode
Development page for SQL mode and companion files.
http://www.geocities.com/kensanata/emacs-sql.html

Will Partain's Collection of Haskell Modes for Emacs
Contains eight different modes. [Dated December 1995].
ftp://ftp.chg.ru/pub/lang/haskell/misc/haskell-modes-9512.tar.gz