Komodo Edit (Windows, Mac, Linux)
It has syntax coloring, folding, background syntax checking, auto-complete and calltips. It also includes libraries for autocompletion for jQuery, Ext JS, Ruby on rails and more.
I just recommend Komoodo Edit.
Aptana Studio (Windows, Mac, Linux)
It has a “code assist” feature where you can find out details about functions, elements, etc.
You can stay synchronized with the server via FTP, SFTP and control your code with the help of an integrated debugger.
There are many other features that will ease the development process including Git integration or a built-in terminal.
Code::Blocks (Windows, Mac, Linux)
A flexible IDE, focused on C/C++ code, that can be extended very easily with its powerful plugin support.
It has syntax highlighting, code folding, auto-completion and a tabbed interface.
There is also a task manager to simplify working on the same project for multiple users.
Eclipse (Windows, Mac, Linux)
Eclipse is a free, state-of-the-art Java development environment (there are also JS, PHP and C/C++ versions).
It has a very active community and many open source plugins + developer tools built around the ecosystem.
CodeLite (Windows, Mac, Linux)
An open source IDE for C/C++ which can be easily extended with plugins.
It has a XML-based syntax highlighting for most of the popular languages and has a powerful code-completion mechanism.
Visual Studio Express (Windows)
Express products are the lighter versions of the Microsoft’s popular developer tools (but they are still powerful).
There are various programs provided under 4 categories:
Each program targets a Microsoft platform (Windows Mobile, SQL Server, etc.) and they are very handy if you’re developing in these platforms.
NetBeans (Windows, Mac, Linux)
It consists of an open-source IDE and an application platform that enable developers to rapidly create web, enterprise, desktop, and mobile applications.
Xcode is the “Mac way” of building for Apple-powered platforms.
It includes features to complete all the build, debug and test cycles.
A graphical editor (Interface Builder) helps designing every aspect of your iOS or Mac OS Xapplication’s graphical user interface.
For iPhone developers, there is a full-featured simulator for a complete testing of the output and much more.
A Java IDE that offers a complete “Mac” user experience with text editors, keyboard shortcuts, toolbars, etc., all behaving exactly as you would expect from a Mac application
There is a powerful code assistant with auto-complete and a library displaying any parameters related to the code.
Geany (Windows, Mac, Linux)
Geany is a simple-yet-powerful IDE that support all major filetypes.
It supports code-folding, auto-completion, code navigation and new features can be added with the plugin support
The editor supports many file types and languages, with syntax highlighting.
There are macros, clip files and templates to automate repetative tasks.
Other features include an integrated HEX editor, FTP client, macro recorder, code explorer and more.
Anjuta DevStudio (Linux)
With a simple user interface, Anjuta has many features including project management, application wizard, interactive debugger, source editor, version control, GUI designer, profiler and many more tools.
Also, the functionality can be extended quickly with thanks to its plugin support.
KDevelop (Windows, Mac, Linux)
The IDE supports many programming languages like C, C++, Fortran, Java, Pascal, Perl, PHP, Python and Ruby.
It has built-in project management, debugger and can be extended with plugins.
Notepad++ is actually an advanced text editor and a must-have for every developer using Windows.
It has syntax highlighters for every popular scripting language and it is usually a perfect fit for a 2nd IDE, for small updates or simpler codes (however, there are many developers who only use it).
Multiple tabs can be created, macros can be used and there are many other features like code folding, advanced search, etc.
Similar to Notepad++, ConTEXT is an advanced text editor where you can work on unlimited files at the same time.
Besides features like syntax highlighting and multilanguage interface, it enables you to store code templates, record macros, compare files and more.
Also, it has customizable help files for each file extension.