Dual N-back is also available for free online at http://cognitivefun.net (under Tests -> Memory) along with other well-known psychological tasks, including visual/auditory reaction time test, Eriksen flanker test, Stroop test, various flavors of the digit span test, paced serial addition test, and more.
Cognitivefun also has a multimodal n-back, where you can customize most aspects of the test (sounds, images, speed run, double or triple n-back, number of trials):
