Among the open-source game engines that re-implement closed-source game functionality and have been talked about recently on Phoronix include Xoreos, GemRB, and OpenMW. A Phoronix reader wrote in this weekend about another such game engine project, Prequengine.
Prequengine sets out to be an engine for Little Big Adventure, an action-adventure game that was released for MS-DOS in 1994. Prequengine is based upon TwinEngine and is being hosted at Google Code. While the project has been worked on from 2010 to 2012, sadly there hasn't been any notable progress since January. The game engine is working, but there's many features not yet implemented.
The benefit of these open-source projects re-implementing proprietary game engines is that they can still utilize all of the original game's content / art assets. One of the major failures of most open-source games have been the games look like shit although there are a few exceptions. Aside from less than stellar graphics, sounds, and other art, most original open-source games lack original creativity when it comes to maps, missions, and other features.