Thief Gold has a new HD texture pack created by Bentraxx, and it really makes the fourteen year old game look a lot better. The game is bit dated, but this definitely makes it look good for its age.
Here's a list of what this mod adds to the game:
I have never gotten around to finishing Thief, but I played quite a bit of it on a friend's PC a few years ago. It's a really awesome game, and this mod might get me to grab it off GOG.com and spend a weekend playing it.
Sharper Shadows: Thief Gold HD Mod Released [Rock, Paper, Shotgun]