in reply to burgermeister

Damn, your definition of old-school is very different from mine.

My go-to free old-school shooter is Freedoom. Completely free assets for the open source Doom engine form a completely free and open source game. They are currently working on giving Quake the same treatment.

Slightly newer are the Unreal games which can be downloaded for free from oldunreal.com/ with the blessings of Epic. I think technically you would also be allowed to get UT3 but nobody wants that.