As you've seen before many times, our choices are usually determined by external factors that we are not aware of. Derren Brown, if you recall, showed us a particularly poignant example with subliminal advertising.
Much more often than we can possibly realize, behavioral economist legend Dan Ariely argues, our choices are determined by factors not simply external to us but downright silly. Using visual illusions as a metaphor for the ways in which our decision-making process can be fooled, Ariely shows a number of fascinating and amusing examples of the quirkly ways in which our brains can be influenced to make choices that we wouldn't ordinarily imagine we'd ever make.
For more, check out the fascinating entries in the free will tag.