Valve debuts Source 2 for free in bid to compete with Epic, Unity

Not entirely sure if it's a fair assessment to say that it was released for free in order to compete with Unreal and Unity.

The general rumblings with Source 2, especially in context of the recent Dota 2 mod tools, was that it would be free. Valve probably made their minds up a long time ago, and GDC is when everyone let it out.