Quake Revitalization Project for map textures and GUI textures

http://qrp.quakeone.com/

Quake Reforged: High Res Skins for Quake

http://quakeone.com/reforged/

Small Mod Compilation. A very good mod, adds/fixes features in Quake. Everything it offers is optional, so, for example, if you don't want new monsters, you can turn them off. And ever other feature too.

http://quakeone.com/forums/quake-mod-releases/finished-works/6076-small-mod-compilation.html

And a whole bunch of other stuff, if you feel this isn't enough

http://quakeone.com/forums/quake-help/general-help/10727-definitive-hd-replacement-content-list.html