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. There's 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 it's being spread like wildfire on the Internet as I am typing this out now. There's still uncertainty whether it is the game's full source code or a part of it, in any case, this could have a huge impact on valve and it's already delayed half-life 2 release date.
News source: Slashdot