franpa: There's a fine distinction between "free" and "Free." Tenebrae is "free," meaning you don't have to pay for it (a/k/a free as in beer). However, due to the bundling of textures and incompatibility with mods, it isn't "Free" (a/k/a free as in speech). Although the engine remains open-source, it uses external files that make it useless otherwise. A shame too, since it was the first Quake mod to be really good-looking. Now, however, DarkPlaces, which someone has previously mentioned, looks better and runs better, and is compatible with most mods.