Quoted from Slashdot:
Pyroman[FO] writes "There is the Half Life 2 source code floating around the net right now. It looks to be about a month old. Theres no official word from Valve on the source code leak yet. Unfortunately those who want to use it to cheat already have it, we need to get the word to legitimate customers to educate them about the situation. "
Surprisingly, this looks genuine and its being spread like wildfire on the Internet as I am typing this out now. Theres still uncertainty whether it is the games full source code or a part of it, in any case, this could have a huge impact on valve and its already delayed half-life 2 release date.
News source: Slashdot