Hosting and domain costs until October 2024 have been generously sponsored by dumptruck_ds. Thank you!
TrenchBroom
From Quake Wiki
Trenchbroom is a recent level editor for Quake programmed and maintained by SleepwalkR. It uses a single 3D view with no 2D views, and boosts some very handy vertex manipulation. Website
See Getting_Started_Mapping for a beginners guide to using Trenchbroom, that also covers a lot of the basics of general Quake mapping.