WinBSPC is a graphical Quake and GoldSrc map decompiler. It takes a compiled BSP file and converts it back to a MAP for use in a level editor. To use it, run the program and locate your map using the "Convert..." option in the file menu.
Please note that the Quake and GoldSrc BSP format does not store the original brush data inside the compiled map. WinBSPC does a decent job of recreating the original brushwork, but differences and broken solids are still very much possible. WinBSPC supports Quake 1, 2, Sin, Heretic II, Kingpin and most GoldSrc games, except for Half-Life: Blue Shift (its maps have to be converted first).
There are a few extra settings available when you select a map to be decompiled. If "best match texturing" is enabled, fewer brushes will be created in the resulting decompile, but the texturing will be less accurate. "no brush merging" simulates the behavior of BSPTwoMap, where a six-sided brush will be split into six separate brushes instead of one brush. Finally, "no liquids" decompiles the map with no water, slime, or lava brushes included.
If "Convert to" is not set, the BSP will be used to generate an AAS file for use with the Quake II Gladiator Bot, so make sure to set it to MAP if you're not working with it. WinBSPC is also capable of reading and extracting BSPs from pak files using the "Extract..." option from the File menu.