NewDark (Thief, Thief II, System Shock 2) ( www.ttlg.com )
This is an interesting one. It's technically a "patch", though lore has it that NewDark was developed using a source leak of the Dark Engine. My best guess is that releasing it as a patch of the original executable, instead of a source port, means less questions and potential legal repercussion.
NewDark is a new game executable for Dark engine games which appeared out of nowhere in late 2012. It was created anonymously, but has been extensively tested and found to be completely safe, to the point that GOG includes it in their System Shock 2 release.
What NewDark does is upgrade the Dark engine to support many new and incredibly great features, such as full DX9 support, high resolution textures, native high resolution, widescreen and multi-monitor support (including extremely high resolutions like triple 2560x1600). All of these features work in Thief 1/Gold, Thief 2 and System Shock 2.
Here's a quick video of it in action with Thief Gold, including HD textures:
They've got the bloom cranked up a little too heavy for my tastes, but that is adjustable.