Edit - $16.5K has been raised so Quake 2 will be free after Quakecon.
Edit - $20K has been raised so Quake 3 will be free after Quakecon
Quake 2 & 3 claim windows are yet to be decided, probably Thursday morning in Australia Quake 2 available 13/8 2am for 72 hours. Quake 3 available 18/8 2am for 72 hours. Source.
Basically log into Bethesda.net Launcher during the 60 hour window to claim the game for free.
Throughout the duration of the show, players will be able to login into Bethesda.net Launcher and claim a free copy of the original Quake on the Bethesda launcher. If charitable donations reach $10,000, players will also be given a free copy of Quake II.