[file format] .BSP is a map file extension used in games such as the Quake series, and games that use the Source engine, such as the Half-Life series and the Portal series. .BSP files use binary space partitioning to generate levels that can be rendered quickly while minimizing the number of polygons that need to be redrawn every time the s... Found on http://en.wikipedia.org/wiki/BSP_(file_format)