The original Thief trilogy is still considered a high-water mark for stealth games by many people, and if you're one of them you probably already know about The Dark Mod. It's the place to go to get your fix of fan missions made in a new engine— specifically, the Doom 3 engine, whose lighting effects are put to good use in maps like those highlighted in our list of the best Dark Mod missions. (In addition to those I'm partial to Creeps, a spooky little horror experience.)
In spite of the name The Dark Mod's a standalone you don't need to own Doom 3 to play, and you can download it for free. It's still being updated today and the latest version is The Dark Mod 2.07. Called "the stability release", it sorts out various problems introduced by the inclusion of things like soft shadows, an uncapped framerate, and multi-core processor support in the previous update. It also sneaks in some new features like faster mantling over small objects, better AI for some very specific situations like when you flashbomb someone who is sitting down, and more you can read about over at ModDB.
It's not really a groundbreaking release, but it's still worth highlighting because well-supported projects like The Dark Mod are one of the things that's great about PC gaming. If you were at all into Thief: The Dark Project and its sequels you should check it out.