Update (January 26th, 2018): The Quake Expo 2011 is now live (again) and integrated it with the current content management system.
Buy Quake from Steam:
http://store.steampowered.com/app/2310/ – Quake 1
http://store.steampowered.com/app/9040/ – Quake 1 Mission Pack 1: Scourge of Armagon
http://store.steampowered.com/app/9030/ – Quake 1 Mission Pack 2: Dissolution of Eternity
http://store.steampowered.com/app/2320/ – Quake II
http://store.steampowered.com/app/2330/ – Quake II Mission Pack: The Reckoning
http://store.steampowered.com/app/2340/ – Quake II Mission Pack: Ground Zero
Buy other id Software games from Steam:
http://store.steampowered.com/sub/440/ – id Super Pack – the id Super Pack includes over 20 games from from id Software, including titles from the series of DOOM, HeXen, QUAKE, Wolfenstein and more!
Get shareware Quake:
Quake 1 (version 1.06)
try the demo, buy the game, get patches