Not that you ever really use the term "high definition" on a PC, but if you did, this mod for the original Deus Ex would surely be described as such.
It's called Deus Ex: New Vision, and has replastered 75% of the walls in the game with remastered, high resolution textures that should make the classic first-person shooter/RPG—which these days isn't looking too good—look a little better.
Note that it's not a complete remaster, so there's no new objects or lighting or anything else. Just new textures for the same old walls that have always been there.
Why would you want to do such a thing? Well, the original Deus Ex is regarded as a masterclass in video game story-telling, and installing a pack like this on a game you can buy very cheaply these days will make it a little easier for newcomers to the series (perhaps spurred on by the upcoming Human Revolution) to digest.