After a development cycle of "only" 19 years, Quetoo has arrived for PC, Mac and Linux:
Website:
https://quetoo.org/news/quetoo-is-here
Github:
https://github.com/jdolan/quetoo
Basically this is an open-source standalone engine/port, featuring faithful remakes of the best multiplayer levels from Quake 1-3. 18 maps are included so far, covering DM, Team DM, CTF, Instagib and Rocket Arena modes. And, most importantly: Fully moddable. Oh, and there's bot support, too.
With any luck the community is picking this up, creating/converting a lot of maps for it and we finally have a sort of freeware Quake 3. Right now it's still a bit buggy and gets updated frequently, so unless you want to help with reporting issues on Github, wait a bit before diving into it.