0xED is a native OS X hex editor based on the Cocoa framework. Features include: Fast editing of large files, unlimited file size, small memory footprint, HFS+ compatible, and more.
Aquamacs is an Aqua-native build of the powerful Emacs text editor. By "Aqua-native," we mean more than just the fact that this version of Emacs runs as a standard OS X application. Aquamacs features extensive customization that enables it to conform better with Apple's standard Human Interface Guidelines (HIG) than standard versions of the editor do.
Atom is a desktop application based on web technologies. Like other desktop apps, it has its own icon in the dock, native menus and dialogs, and full access to the file system. Read more Killer App
CocoEdit is a pure cocoa rich text editor.
CPP Edit is a simple C and C++ editor,
Daktari brings a familiar metaphor to managing web site projects with unique features for editing web pages. As web site projects are added to Daktari's Library, all of the web pages and image files are listed in a Library List view. Double-clicking a web page in the Library List view opens the page into Daktari's Head and Body views for editing.
Eddie is a programmer's editor for Mac OS X and Linux, inspired by MPW.
Editra is a multi-platform text editor with an implimentation that focuses on creating an easy to use interface and features that aid in code development. Currently it supports syntax highlighting and variety of other useful features for over 30 programing languages.
Emacs on Aqua
GNU Emacs ported to OS X and GNUstep, based on the earlier Rhapsody, OpenStep, and NeXTstep versions. This port offers superior platform support for MacOS X and GNUstep, including full desktop integration (dock, drag/drop, etc.), standard keybindings, use of standard Font, Color, and File open/save panels, and support for Services. Via GNUstep, it provides anti-aliased font rendering on Linux and other Unix systems.
Enhanced Carbon Emacs
Enhanced Carbon Emacs is based on the standard emacs-21.3.50 CVS distribution, enhanced with many packages, including a fully customisable LaTeX editing environment based on AucTeX, RefTeX and other packages.
Hex Fiend is a fast, open source hex editor. It can work with huge files, has a small footprint, with smart savings, and a data inspector.
HexEdit is a hexadecimal file editor for the Macintosh. HexEdit can handle file sizes of up to four gigabytes with ease, yet has memory requirements of less than a megabyte (with colour turned off). It can also compare files for similarities or differences.
Light Table is a new open source interactive IDE written in ClojureScript that lets you modify running programs and embed anything from websites to games. It provides the real time feedback needed to not only answer questions about our code, but to understand how programs really work. Read more
MacNote is "Notepad" for Mac OS X. You can use it just like the Notepad application in MacOS9, and you can use also it with toolbar and drawer in Mac OS X style and you can paste images to make it work like "Scrap book" too.
Vim is an almost compatible version of the UNIX editor Vi. Many new features have been added: multi-level undo, syntax highlighting, command line history, on-line help, spell checking, filename completion, block operations, etc.
NotePad is the Notepad replacement you've been looking for since converting from 9. NotePad can import your Classic Notepad file so you don't lose that critical data.
Nvu (pronounced N-view, for a "new view") is a complete Web Authoring System that combines web file management and easy-to-use WYSIWYG (What You See Is What You Get) web page editing. Nvu is designed to be extremely easy to use, making it ideal for non-technical computer users who want to create an attractive, professional-looking web site without needing to know HTML or web coding. Read more
(commercial) Pacu is a lightweight source code editor. It provides a responsive user interface with a low memory footprint. All functions are fully keyboard accessible and have been designed to fit on relatively smaller screens (>=1024x600).
Plain Text Editor
Plain Text Editor allows you to edit plain text files with many options. It works with any kind of plain text files (text, source code, HTML, ...), and can automatically concatenate documents by dropping them on the same window.
PLCEdit is a cross-platform editor for POU files (program organisation unit). Supported POU sources are PRG, FB, Functions and Global Variable lists, according to the IEC61131-3 standard.
Poedit is cross-platform gettext catalogs (PO files) editor which runs on Unix, Windows and Mac OS X. Unlike other editors, Poedit provides compact view of data and very effective UI.
Sdef Editor is a "Scripting Definition File Editor". If you are developer, and you want that your application be scriptable, you have to write a AppleScript dictionary, or terminology. Sdef Editor can create, and edit sdef xml files.
Mac OS X Sieve Mail Filter editor and client
(commercial) skEdit is a text editor for Mac OS X (10.2 or later), with a focus on web design and development.
Smultron is a text editor written in Cocoa for Mac OS X with many of the features that you need.
(commercial) Sublime Text is a sophisticated text editor for code, markup and prose.
(commercial) TextMate is not an IDE but by using its powerful snippets, macros, and unique scoping system, it can often provide features that even a language specific IDE lacks. It has enough project management features to keep most users happy, but is otherwise kept lightweight with a clean and minimalistic GUI.
(commercial) a general purpose text editor, and Unix and server administrator's tool
texty is a text editor featuring autoindent, basic syntax highlighting, and other features.
Visual Studio Code
Build and debug modern web and cloud applications. Code is free and available on your favorite platform - Linux, Mac OSX, and Windows.
VoodooPad Lite is the stripped down free version of VoodooPad.
Yet Another Carbon Emacs Distribution: provides individual Apple Installer packages of Carbon Emacs for Mac OS X 10.3 and other emacs packages for installation into Carbon Emacs.