(September 16, 2017)
BSPSource is a Java-based graphical map decompiler for all Source games, up to Dota 2. It converts a compiled BSP into an editable VMF file for you to use in Hammer. It's based off of the earlier, now obsolete, VMEX. To run it, run
bspsrc.bat and drop BSPs into the box, then click "Decompile".
Decompiled map names have
_d appended to differentiate them from the original. Decompiling maps with BSPSource may not always give you a perfect VMF. Some differences like keyvalues, materials, and other variables may occur. Some solids, instances, and areaportals may also be broken.