While we're on the topic, you can get the Halflife Pro SDK at:
http://www.valvesoftware.com/hlsdk.htm
Which supposedly has everything you need to compile halflife from scratch.
You have to fill out an NDA, other restrictions, but suprisingly you can sell anything you make over the web, just not on CD's floppys etc.
I haven't ordered it yet, though. If I do I'll never finish the project I'm working on now (game written from scratch) and I'm so close.