Do you like the Thief Games? I like the Thief Games, but The Dark Mod was made by people who love the Thief Games, so they decided to make their own version of them, but update them a bit using the Doom 3 engine. Things really went their way when Id Games released the Doom 3 Engine as open source, so now you can download and play The Dark Mod for free on Windows and Linux, and you don’t have to install Doom 3. They really should change the name of the game now that it is no longer a mod.
If you have never played Thief than you are missing out. The game is a first person stealth game where you club people on the back of the head or shoot people from the darkness with your bow and arrow and of course steal stuff. All based in kind of a steam punk dark ages world. The Dark Mod is exactly the same, but with a minor difference.
The Thief games always had an overarching story, but The Dark Mod is a series of fan created missions that you can download in game, or find them on the forums. They also encourage you to make your own missions and share them with the community, so there is a lot of content.
As you can guess, I think this is a pretty great game, and since it is free and it runs on the older Doom 3 Engine there is no reason not to download it and see if you like it as well.