There are cool features like Deluxe mapping, Parallax Occlusion Mapping, motion blur, lens flare and many more, it's all very modern and runs great.
I like this mod over others as it strives to improve the graphics quality without changing the art style of the original game to much. He started it years ago and has continuously added and refined it's feature set. This is the latest version of Kirk Barnes Engine Mod, Quake 2 XP. EDIT: There is an updated version on the Sourceforge website, these instructions have been deprecated.
