[erikj]

It would be amazing if Valve made Source Engine open source, but that would require them to replace Havok and perhaps other middleware.

[foxhop]

Many games from this era were free homebrew Half-life mods. I think Valve should do more to promote hacking and modding their games.

Counter strike was created as a mod by the community as well as day of defeat.

[talmand]

I do believe that almost all of Valve's games started out as mods.

One could make an argument that all of their games started out as mods, in one way or another.