all of the FPS games raven made are editable... hereticII, SOF, heretic, hexen... elite force, you just have to look for in what department you want to edit in.
coding? you better get the games SDK
mapping? most of the SDK's includes the map making tool they use... or either use QOOLE, worldcraft, GTKRadiant, Q3radiant, etc.
try searching the *gasp* planet sites like planetquake.com, raven-games.com, planetsof.com for more info.