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Source 2 is Coming and It's Free
Valve officially announced Source 2 at GDC this year, and they're giving it away for free:
The value of a platform like the PC is how much it increases the productivity of those who use the platform. With Source 2, our focus is increasing creator productivity. Given how important user generated content is becoming, Source 2 is designed not for just the professional developer, but enabling gamers themselves to participate in the creation and development of their favorite games. We will be making Source 2 available for free to content developers. This combined with recent announcements by Epic and Unity will help continue the PCs dominance as the premiere content authoring platform.