With the release of HL2 approaching, I thought some of you might enjoy this old (ancient) article about how Valve went about designing the original Half Life. Word is that they still use the same 'Cabal' system when working on HL2.
http://www.gamasutra.com/features/19991210/birdwell_pfv.htm
http://www.gamasutra.com/features/19991210/birdwell_pfv.htm