TrenchBroom

TrenchBroom is a free (GPLv3+), cross platform level editor for Quake-engine based games. It supports Quake, Quake 2, and Hexen 2 and runs on Windows (XP and newer), Mac OS X (10.6 and newer) and Linux. TrenchBroom is easy to use and provides many simple and advanced tools to create complex and interesting levels with ease.