ANTLR Mode: Major mode that provides various features for editing ANTLR grammar files.
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.
CC Mode: The development homepage of the Mode for editing C/C++ and other languages with similar syntax.
David Ponce's Emacs Page: Additions to JDE.
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]
Emacs Development Environment Tools: CEDET is a collection of tools written with the end goal of creating an advanced development environment for Emacs users.
Emacs Major Modes and Packages: Bison and Flex mode can be found there.
Emacs Rexx Mode: The current version of the Rexx mode for Emacs with complete font-locking and a Rexx-debug minor mode.
Haskell Mode for Emacs: An Emacs mode currently supporting font locking, declaration scanning, documentation, indentation, hugs interaction.
Hugs mode for Emacs: Provides fontification and cooperation with Hugs -- by Chris Van Humbeeck.
Ilisp: Integration of various Lisp implementations (mostly Common Lisp systems and various Scheme dialects) within Emacs.
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.
PCL-CVS: Emacs interface to CVS, the open source Concurrent Versions System.
Phillip Lord's Emacs Packages: Mostly additional packages for JDE.
PHP Mode: Sourceforge project page of a major mode for editing PHP source.
Python Mode: Python support for Emacs and XEmacs.
SQL mode: Development page for SQL mode and companion files.
Will Partain's Collection of Haskell Modes for Emacs: Contains eight different modes. [Dated December 1995].