Tools

• XNA Game Studio 3.1 - the game development framework based on .NET for game development for Windows, Xbox 360 and Zune.
• Dark GDK Game Development Kit for Microsoft Visual C++ Thanks tosome_kid
• Unity Game Development Tool (Thanks to lesPaul456)
• FlatRedBall Game Engine(Thanks to lesPaul456)
• Blender - a free, open-source, 3D design software. Useful for creating 3D models.
• Softimage Mod Tool - a free tool for creating 3D models. Supports exporting to XNA.
• Paint .NET - a simple graphics tool that will let you create basic textures and sprites.
• GIMP - a free, open-source, graphics tool that can be used for creating more complex textures and sprites.
• InkScape - a vector graphics editor. (thanks for mentioning this product to modi123_1)
• SpiralGraphics Tools - a set of tools for creating textures. (thanks to SixOfEleven)
• Anim8or - a free tool for 3D modelling (thnaks tostayscrisp)
• FMOD - a great utility to work on the sound for your games (link by chuckb)
• SDL Mixer - a free audio mixer library (link shared bystayscrisp)
• Linux Multimedia Studio - a free application to produce music and sound effects (link shared by X@MPP)
• MaPZone - texturing tool (link by X@MPP)
• Sprite Animating ToolThanks to Hellbroth
• Alice - 3D Programming Environment (Thanks toNeoTifa)
• WavePad Sound Editor - Free Sound Editor with library of free sounds/music (Thanks to SixOfEleven)
• SCIRRA - Create games effortlessly (thanks tomodi123_1)
• Craft Studio - Make games together with Craft Studio (thanks to modi123_1)
• Stencyl - Cross-platform game making software (thanks to modi123_1)
• Flixel - An open source game making library (thanks tomodi123_1)
• ShoeBox - A sprite base drag-n-drop editor (thanks torenderhjs)
• DAZ Studio - Figure customization, posing, and animation tool (thanks to¿serenity?)
• Fraps - Provides you with Frames Per Second(FPS) on any DirectX or OpenGL games / applications (thanks toAaron1178)
• Cocos2d-x - A cross-platform tool chain (thanks tosparkart)
• CryEngine - The Cry Engine editor (thanks to XeVile)
• http://pixologic.com/
• https://www.unrealengine.com
• http://icofx.ro/
• http://www.yoyogames.com/studio
• http://haxe.org/
• http://www.openfl.org/
• http://bigworldtech.com
• https://www.loomsdk.com/How to
• http://www.babylonjs.com/

Textures

• KatsBits TGA Textures - some free textures in TGA format for your game.
• CG Textures - a huge collection of various textures. All textures are sorted into categories so it's easy to find exactly the one you need.
• TextureKing - also an extensive library of free textures for your game.
• texture lovers - a website with references to lots of different free textures for your game.
• Image * After - a library full of free images and textures.
• BackgroundLabs - a set of nice textures and backgrounds.

Sprites

• VideoGameSprites - a library of simple 2D sprites.
• Spriters Resource (thanks tostayscrisp)
• Charas Project - a large collection of sprites (thanks togabehabe)
• The People's Sprites - a great resource with free sprites (shared by gabehabe)
• Sprite Database - a large collection of free sprites (link by gabehabe)
• NES/SNES Sprites (link bygabehabe)
• RPG2000 - site in German (game resources - link bygabehabe)
• SpriteLib GPL (link bygabehabe)
• Last Guardian Sprites (link by SixOfEleven)
• Lost Garder - Some excellent sprites (thanks to Stayscrisp)
• http://hasgraphics.com/
• http://opengameart.org/

SDKs

• DirectX SDK - The latest DirectX SDK release
• Open GL - The OpenGL official site for SDK releases
• Moai - An IOS and Android open source SDK (thanks tosparkart)
• http://pygame.org/

Models

• Psionic3D - some free models for your game.
• 3DGuru - a collection of 3D models for non-commercial use.
• 3D Ocean - 3D models for purchase (thanks toAaron1178)
• The 3D Model Store - Purchasable 3D models (thanks to the3dmodelstore)

Music

• Nosoapradio.us - Music for your game (thanks toStayscrisp)
• https://lmms.io/ orhttp://lmms.sourceforge.net/

Sound FX

• FreeSound (by Hellbroth)
• ZeldaSounds (by Hellbroth)
• PacDV Sounds - a large collection of free sounds (sorted by categories).
• SoundJay - a large set of free sounds (sorted by categories).
• Bfrx - Make sound effects for you game (thanks to Kilorn)

Sites With Mixed Resources (have different types of resources)

Fonts

• DaFont - a resource with lots of free fonts (thanks togabehabe)
• 1001 Free Fonts - again, a resource with lots of free fonts (mentioned by gabehabe)
• UrbanFonts - yet another free font resource (shared bygabehabe)
• Acid Fonts (shared bygabehabe)
• Font River (shared byVulcanDesign)

Backgrounds

• Background HQ - a collection of game backgrounds (link bygabehabe)

Articles & Blogs

• Game Engine Anatomy 101- A great article, found byAaron1178
• 3D Total Games - Game theory and concept blog (thanks to modi123_1)
• Optimal convex decompositions - (thanks toAaron1178)
• So you want to be a game developer, now what? - A great article found by Serapth
• Game Programming Patterns - common patterns. (found by modi123_1)
• A Software Architecture for Games

Documentations & Tutorials

• Fixel API Documentation - Flixel's official API documentation (thanks tomodi123_1)
• Learn how to Flixel - A Flixel tutorial (thanks tomodi123_1
• 40+ Fantastic Game Development Tutorials From Across the Web - Thanks to modi123_1
• Shadow Volume Tutorial - How to produce Shadow Volumes in 3D scenes (thanks to BBeck)
• GameDevelopment Tutorials - .tutsplus.com - Extensive list of Game Development tutorials (thanks to modi123_1)
• Les Forges Pixel Art Course- For anyone beginning spite making (thanks to evilyoshi)
• Rastertek - A great collection of DirectX10, DirectX11, OpenGL and other assortments of tutorials (thanks to Aaron1178)
• Building a simple yet powerful MMO game architecture - Part 1: Introduction
• Building a simple yet powerful MMO game architecture - 2: Gaming and Web integration
• Building a simple yet powerful MMO game architecture - Part 3: Capabilities and limitations
• http://www.swiftless.../mathstuts.html
• http://www.essential...om/tutorial.htm
• http://gamedevelopme...--gamedev-12718
• http://warriors.eecs.../quakefinal.pdf

Books

• Real-Time Rendering, Third edition(Amazon Link) - A well versed book on Real-Time Rendering (thanks toAaron1178)

Misc

• http://www.gamecareerguide.com/- Career direction, understanding, etc
• http://www.gamasutra.com/- crazy good blog on all things game dev related
• http://www.pixelprospector.com/