With GTKradiant you can make bsp's. (QuakeIII maps, also used in Return to Castle Wolfenstein, KingPin,...)

wikipedia:
.BSP is a map file extension used in games such as the Doom, Quake, and Half-Life series and other games. .BSP files use binary space partitioning to generate levels that can be rendered quickly while minimizing the amount of polygons that need to be redrawn every time the screen refreshes. One of the biggest problems with the .BSP file format is that large open areas don't work well due to the nature of the partitioning algorithm used.

tutorial