LiveCode Forums.

Runtime Revolution - Cross-platform development environment
GIMP - graphics editor
Audacity - recording and editing sounds
FileZilla - FTP, SFTP client

Mac OS X

VMWare Fusion - Virtual machine to run Windows and Linux
Querious - MySQL database management
DMG Canvas - Builds disk images from template documents
Screenflick - High performance screen capturing
Coda - Web development application
Cyberduck - FTP, SFTP, WebDAV browser
Apple Development Tools - Script Editor, Interface Designer, XCode, Package Manager, Icon Composer, ...
Pacifist - Analyze packages
Pixelmator - Image editor

Windows

Paint.net - graphics editor
IcoFX - freeware icon editor
InnoSetup - free installer for Windows programs
AutoIt - automation language for Microsoft Windows
Microsoft Development Tools - Visual Studio 6 (Visual Basic, Visual C++), Visual Studio 2008 Express (Visual Basic .NET, Visual C++)
Intype - code editor
Res Hacker - utility to view, modify, rename, add, delete and extract resources in executables

Linux

Anjuta - IDE for C, C++

Web:

On-Rev - Been programming for years in numerous situations. I have to say that the combined strength of Rev & On-Rev is wonderful!
PHP - Still need to translate my existing pages to .irev, just been too busy.

OS X:

Runtime Revolution Studio - For OS X, Windows Dev (Linux may be an option soon as well)
Unity 3D - For game programming, sadly this has been put on hold due to a current project taking precedence.
VMWare Fusion - Running: Windows XP & Windows 7
Pixelmator - Graphic Editor
ForkLift - FTP & File Browser

Windows:

Basic4PPC - For Pocket PC developement
IcoFx - Icon Editor

Was using HanDBase for PPC quick dev, but it is WAY to limited for what I need, and the price on Basic4PPC is an incredible value. just like Rev is.

With Revolution & Basic4PPC, you can cover programming for almost any situation.

Next step will be the iPhone SDK!

Just a few..

Re Visual Studio, I use it when I have to, but I've started doing most of my non-rev Windows work in CodeBlocks (http://www.CodeBlocks.org), an open-source C++ IDE without all the bloated overhead of VS.

@xeir
The iPhone SDK is on my list too. Did you evaluated Torque by GarageGames before you choosed Unity3D?

Robert

Revolution - Of course

Mac OS:
XCode - For iPhone application development.

Windows:
IcoFX - Free editor for both Windows (.ico) and Mac (.icns) icons.
GIMP - GNU Image Manipulation Program
InnoSetup - Free setup maker.
AdvancedInstaller - Full-pledged maker for Windows Installer Packages (.msi)
TransMac - A windows app that you can use to make DMG files for Mac.
VMWare Workstation - A workstation emulator.
Resource Hacker - View, modify, remove resources from a DLL or a program.

Linux (Ubuntu)
Still getting used to it.