A standard introductory graduate textbook on game theory, A Course in Game Theory by Martin Osborne and Ariel Rubinstein is now freely available in PDF format.  You can download it here.  This is a great step toward the day when there will be top quality freely available textbooks in all subjects and the day when students and faculty will not be held-up.  While we are on the subject, here is a list of other free economics books  (right-hand column.)