Quake 3 is currently free-to-keep on the Bethesda launcher, but only if you log in before the promotion expires. That's in roughly three days: it'll expire on August 20 at 9am PT, 12pm ET, 5pm BST and August 21 at 2am AEST.
The 1999 classic is a multiplayer-only affair, so if you're keen on a single player campaign hopefully you picked up Quake 2 while it was free. That said, the Quake modding scene is still active, and if you look around you'll find some decent single player total conversion mods.
2020 is a golden age for free-to-keep games. Remnant: From the Ashes is currently free on the Epic Games Store. For a list of what else is going for zero cents, here are all the free games you can grab right now.
The Quake giveaways come in the wake of QuakeCon At Home, where Bethesda promised it would give away Quake 2 and Quake 3 if a charity fundraising milestone was reached. The goal was $30,000, and that was met and exceeded by $686 bucks.