Quake Reforged is a project being ran mainly by Nergal and Alfader to create high-resolution textures and re-skins for monsters and game levels in Quake, along with Dissolution of Eternity and Scourge of Armagon.
New features such as highly updated shaders and new graphical systems are present and currently available for download for people who own the full version of Quake and/or its expansions.
Most of the downloads are applicable to the two main Quake graphical engines: Darkplaces and Qrack. Tenebrae may also be available. Many of the skins and textures have alternate versions, so some people may prefer different versions of the textures/skins. A link to the homepage of the Quake Reforged Project can be found here below.