Xash 3D[engine]

Looking at it's source code I can see that it's just a straight-up HL mod, so it meets most of the criteria and is just violating by making the source available. I'd guess that either Valve have no idea it exists or they don't care too much about that kind of violation. I wouldn't rely on them not caring about replacement engines or running HL content in other engines though (anybody remember CSOS from the old QER days?)