Starting on Monday, August 17th at Noon PST on the official Avatar website - AvatarMovie.com - Fox will be giving away first-come, first-serve tickets for free. There will be two screenings on Friday, August 21st (one at 6:00PM, the other at 6:30PM) in over 100 IMAX 3D theaters around the world. I don't have exact details on which theaters will be involved, but it looks like it'll only be in IMAX, so you better find the nearest one now. These tickets will probably go very fast on Monday.