The Doom reboot, which is great, is free on Steam until midday tomorrow (EST).
Meanwhile, the reboot of Shadow Warrior, which is also good, is free from the Humble Store for another few hours.
I'll add that Doom actually runs on my aged computer (2600K, geforce gtx 570, 8GB ram). It can lag some when it's loading a new area, but that hasn't been too big of a deal.
I really hope Vulkan takes off.