System utilities
XEmacs
XEmacs is a highly customizable open source text editor and application development system. It is protected under the GNU Public License and related to other versions of Emacs, in particular GNU Emacs. Its emphasis is on modern graphical user interface support and an open software development model, similar to Linux. XEmacs has an active development community numbering in the hundreds, and runs on Windows 95 and NT, Linux and nearly every other version of Unix in existence.
Website: http://www.xemacs.org/
SharpDevelop
#develop (short for SharpDevelop) is a free IDE for C#, VB.NET and Boo projects on Microsoft's .NET platform.
website: http://www.icsharpcode.net/OpenSource/SD/
Notepad ++
Notepad++ is a free source code editor and Notepad replacement that supports several languages.
Website: http://notepad-plus.sourceforge.net/uk/site.htm
jEdit
jEdit is a mature programmer's text editor with hundreds (counting the time developing plugins) of person-years of development behind it.
Website: http://www.jedit.org/
SciTE
SciTE is a SCIntilla based Text Editor. Originally built to demonstrate Scintilla, it has grown to be a generally useful editor with facilities for building and running programs. It is best used for jobs with simple configurations - I use it for building test and demonstration programs as well as SciTE and Scintilla, themselves.
Website: http://www.scintilla.org/SciTE.html
Python
Python is a dynamic object-oriented programming language that can be used for many kinds of software development.
Website: http://www.python.org/
Ruby
A dynamic, open source programming language with a focus on simplicity and productivity.
Website: http://www.ruby-lang.org/en/
Qt
Qt is a cross-platform application and UI framework. Using Qt, you can write applications once and deploy them across many desktop and embedded operating systems without rewriting the source code.
Website: http://www.qtsoftware.com/
KDevelop
The KDevelop-Project was founded in 1998 to build up an easy to use IDE (Integrated Development Environment) for KDE. Since then, the KDevelop IDE is publicly available under the GPL and supports many programming languages.
Website: http://www.kdevelop.org/
Anjuta
Anjuta is a versatile Integrated Development Environment (IDE) for C and C++ on GNU/Linux. It has been written for GTK/GNOME and features a number of advanced programming facilities including project management, application wizards, an interactive debugger and a powerful source editor with source browsing and syntax highlighting.
Website: http://www.anjuta.org/
GTK+
GTK+ is a highly usable, feature rich toolkit for creating graphical user interfaces which boasts cross platform compatibility and an easy to use API. GTK+ it is written in C, but has bindings to many other popular programming languages such as C++, Python and C# among others. GTK+ is licensed under the GNU LGPL 2.1 allowing development of both free and proprietary software with GTK+ without any license fees or royalties.
Website: http://www.gtk.org/