Game Development Tools
EditGraphics Editors
Gimp
An advanced bitmap based image editor that has many capabilities similar to commercial software. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.
Paint.NET
With an easy-to-learn but powerful set of tools, Paint.NET can be used for photo retouching, tilemap editing, pixel art editing, and so on. It contains advanced features such as levels and curves editing, noise addition and reduction, fractal and Perlin noise generation and more, but can also be used as a simple image editor.
Inkscape
An advanced vector image editor with an advanced editing toolset. Inkscape has capabilities similar to commercial software such as Adobe Illustrator, CorelDraw, or Xara X. Inkscape leverages the W3C standard Scalable Vector Graphics (SVG) file format. Inkscape supports many advanced SVG features (markers, clones, alpha blending, etc.) and great care is taken in designing a streamlined interface. It is very easy to edit nodes, perform complex path operations, trace bitmaps and much more.
Krita
Krita is a free digital painting and illustration application. Krita offers CMYK support, HDR painting, perspective grids, dockers, filters, painting assistants, and many other features you would expect.
Sound Editors
Audacity
Audacity is free, open source, cross-platform audio software for multi-track recording and editing.
LMMS
LMMS is a free cross-platform software which allows you to produce music with your computer.
SFXR
SFXR is a sound generator, widely used for creating sound effects for Ludum Dare games.
BFXR
BFXR is an improved ActionScript 3 port of SFXR with more features. It has moved in the direction of increased complexity and range of expression.
LabChirp
LabChirp is a program for creating sound effects.
Map Editors
Tiled Map Editor
Tiled is a general purpose tile map editor. It is meant to be used for editing maps of any tile-based game, be it an RPG, a platformer or a Breakout clone. This demo shows how to load a Tiled map.
Ogmo Editor
Ogmo Editor is a generic level editor for indie game developers who use Windows. It also happens to be free and open source. The editor is built to be reconfigurable, so you can set it up to work well for your game project. This HaxeFlixel tutorial demonstrates how to create a simple top-down level with Ogmo.
Asset Utilities
ShoeBox
ShoeBox is a free Adobe Air based app for Windows and Mac OS X with game and UI related tools. It has many utilities that make asset management easier, such as sprite packing, sprite extraction, texture ripping, bitmap font creation, and more.
Texture Packer
Texture Packer does one thing very well - it packs several sprites into one large image to help reduce load times for your game. It's easy to use and easy to integrate with HaxeFlixel. This demo shows how to use a TexturePackerAtlas.
More Resources
Ludum Dare List
Ludum Dare hosts a great list of tools for general game development.
PixelProspector
Huge list of resources for game dev. A must read for indie developers.