i would have to second #16’s post in say autocad is somewhat similiar, but not really.
i have just come across what appears to be a good .bsp decompiler. here’s the link:
http://www.geocities.com/cofrdrbob/vmex.html
then some other random tools that are mainly used for modeling (which i don’t do but if anyone cares to):
http://www.chaosincarnate.net/cannonfodder/cftools.htm
btw, hammer map editor is downloaded with the source sdk, so if anyone cares to try their hand at mapping…