
Tiled is a general purpose tile map editor. It’s built to be easy to use, yet flexible enough to work with varying game engines, whether your game is an RPG, platformer or Breakout clone. Tiled is free software and written in C++, using the Qt application framework.

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The guys at xkcd.com took the time to make a “world map” of online comunities..
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An Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using 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. We also aim to maintain a thriving user and developer community by using open, community-oriented development.
Available for Windows, Linux and Mac Os X.

Trukz (pronounced “Trucks”) is a free persistent browser-based trucking simulation game designed to let you try your hand at being a truck driver hauling loads to and from various cities around North America, Europe, and Australia. There are over 35 cargo types in the game each with its own driver skill requirements so take the little jobs early on to get started and work your way to become the best driver making the most money in the game.
Purchase a truck and keep it running in top condition and full of fuel while fighting fatigue, severe weather, the police, and mechanical failures as you drive your route to complete your haul on time.

Nexuiz is a free open-source first person shooter that runs on Windows, Linux and OSX.

This mappack includes ALL 40 levels from WCS’s Flash version, additional bonus levels and features. All together, we’ve got here over three and a half hours of amazing gameplay.
Requires Steam with Half-Life 2 and Portal installed in order to play