Valve Map Extractor (VMEX) is a decompiler for Source engine maps. It converts BSP format files back into VMF format maps that can be loaded into Hammer. Since version 0.98g it supports all of Valve's games.
Left 4 Dead
A hacked version of vmex was created for decompiling Left 4 Dead maps by TerabyteDragon. This hack resolves an issue with static props (specifically how large the entries are in the BSP file). This version can be downloaded from the link below. Note: This version of vmex is NOT guaranteed to work with maps from any other game.