Open source text editors, hex editors, etc.
Please indicate (in round brackets) the programming language, and [in square brackets] the operating system that the code is known to work on.
To quote the Emacs Manual: "Emacs is the extensible, customizable, self-documenting real-time display editor." Emacs, being one of the flagships of the Free Software Foundation is used and developed by countless people for many years. It is available for a big number of OS and due to its extensibility it is like a swiss army knife for all editing tasks.
Please submit only sites and pages of general interest for Emacs users to this category. Modes and packages for Emacs should be submitted to the appropriate subcategory or Emacs Lisp.