Run down of some of the best programming tools that I use or find exceptional. All are GPL or similar open license.

1.) Aptana Ajax/Web IDE

if you want to do Javascript/Ajax web development you must take a look at Aptana. This is a full featured IDE with built in Java, Web server, complete with code completion. Supports PHP, Javascript, HTML, CSS, and XML with complete syntax highlighting, debugging, etc. Runs on Linux, Windows, and OS X. Installing on Kubuntu Linux was a tad annoying even using the instructions on the net. Email if you need assistance on Ubuntu/Kubuntu or go through the instructions here for an apt-get install. You may still need to set this variable, export MOZILLA_FIVE_HOME=/usr/lib/mozilla, if you get a libc.so.6 error and Mozilla needs to be installed. I didn't use the apt-get install so I don't know what problems may arise.

1a.) ActiveGrid web 2.0 development Another really cool IDE for modern web development. The best database support I've seen in an open product. It has self contained Python, Java JDK, Apache all built into the IDE, which do not affect other installs of these products! Supports Python, Java, and PHP with auto completion, highlighting, etc. Licensed under the Apache 2.0 license. Runs on Windows and Linux.

2.) SE|PY Action Script IDE

If you develop in Flash and you need a more powerful ActionScript editor you should look at SE|PY which sports syntax highlighting, code completion, class browser, and even a swf compiler via MTASC. It runs on Windows, which includes an installer, but It should compile on Mac OS X. At present there isn't a OS X binary.

3.) Eclipse Java IDE

If you're a Java developer you might want to look at this great IDE. Eclipse is unique as it has a great plug-in architecture, which really extends it's use. There are currently many great plug-ins like Laszlo Python/Jython C++ Javascript etc. Eclipse has all the great features of any professional IDE and runs on Linux, Windows, and OS X and requires Java.

4.) NVU Web Authoring System

If you want to create simple web-sites or fast templates with HTML and CSS using a more visual WYSIWYG approach NVU is for you. It's similar to Front Page, has syntax highlighting and auto completion, and sever connectivity for publihing. NVU runs on Linux, Windows, and OS X.

5.) SPE Python editor

The actual site is down and it appears a new on is being developed here. This is one of many great Python IDE's. The great thing about SPE is that runs on Linux, Windows, and OS X. Spe has code completion, syntax highlighting, class explorer, debugger, as well as wxGlade GUI designer. It even supports Blender3D scripting.

Being a Python programmer the majority of the time I must mention my other favorites.

5a.) BOA Constructor is a great Python application development environment for designing GUI driven applications in Python using wxWindows. Linux, Windows, OS X.

5b.) PyScripter is my favorite Python IDE on Windows! Easy to install, tons of great features similar to SPE's features mentioned above, but feels more like a Windows application to me than SPE.

5c.) Eric is my editor of choice on Linux where I spend most of my time; although, it does run on Windows and OS X as well. It also has built in GUI tools and designers using QT and has all the features found in PyScripter and SPE.

6.) Bloodshed Dev-C++

The best open source GCC based C++ IDE for Windows in my opinion. Code completion, syntax highlighting, class explorer, debugger, as well as GUI design. Easy Install.

6a.) SharpDevelop C# and VB.Net If .Net is your thing here's a great free IDE for C# and VB.Net. Greate features including
Window forms design, auto completion, higlighting, etc. Also see Mono C# for other platforms.

6b.) Kdevelop on Linux is a great IDE. I don't do much C/C++, but Kdevelop is really nice and has all the great features found in professional programming environments. Kdevelop is a KDE application, but runs fine in Gnome. There may be other really good C/C++ editors for Linux, I am sure there are, but I am not aware of any other than Anjuta DevStudio.

6c.) Xcode is included with OS X, which is one hell of a IDE! It isn't C/C++, but close using Objective C. The GUI creation tools are top notch and Objective C is a great language. The learning curve is strange if your coming from Windows Visual Studio, but worth it if you want to write OS X native applications.

7.) RadRails Rudy on Rails IDE

A full featured IDE for Rails development. IF you're working with Ruby on Rails this will greatly speed up your programming. Has all the great features seen in the other IDE's on this page and runs on Linux, Windows, and OS X. It is built on Eclipse and also available as an Eclipse plugin.

I must mention at least one Firefox extension that just rocks if you're doing Javascript programming. Firebug makes life a little nicer indeed!

There are so many programming languages to choose from and to many great resources to list here. Check my free software list for even more resources.