NetRadiant is a free and open-source 3D game level editor using the brush methodology.
NetRadiant is cross platform and is known to run well on Linux, Windows and FreeBSD. While not being native on macOS yet, it is reported to run well on this system using Wine. Download NetRadiant and start mapping!
Historically a fork of the GtkRadiant project from Id Software, it is maintained by a community of gamers, mappers, modders and game developpers.
NetRadiant supports many other games! Check it out!