Wow. I am constantly amazed at the ambition of open source developers working for free. Below is a video from the OpenMW people, who are attempting to create an open source version of Morrowind (Elder Scrolls). They are taking a commercial game they love and implementing it from the ground up in a new engine. So all the coding needs to be done, but the art assets, writing, quests and so forth can be taken from the normal game. All so we get a version that can easily run on any operating system, with a better interface, more hackability and prettier graphics.
I hope they the same with the oblivion engine so I can run Nehrim on it (and Fallout while we are at it).