I am pretty sure that the Quake 3 demo was released for Linux first and my memory is that this was an intentional move by Carmack to promote Linux [1]. 1. Wiki

I am pretty sure that the Quake 3 demo was released for Linux first and my memory is that this was an intentional move by Carmack to promote Linux [1]. 1. Wikipedia lists the release dates for the full game, which were in Dec 1999 for both Windows and Linux (windows a few weeks earlier). I don't recall exactly when the demo came out, but I think it was sometime in the winter of 1999.