Widelands is an open source (GPLed) real-time strategy game. It is built upon the SDL and other open source libraries and is (and will always be) under heavy development. If you knew Settlers I & II™ (© Bluebyte), then you already have a rough idea what Widelands is all about because Widelands is heavily inspired by those two games.
User summary
Although first build of Widelands was released in late 2002, it has always been a work in progress: Ten years later, Build 17 would see its release. This game provides players with a relaxed-pace, but real-time strategy experience. The focus on the game is in strategic placement of your roads and buildings, in order to maximize access to resources, space for farms, and efficiency of your transportation infrastructure. As you expand your borders, you will encounter your enemies, and soon thereafter, blood will be shed.
Source: The Game Website.