Technically, AutoStitch is just a demonstration of panoramic stitching technology that the developers license to other commercial applications. But you can ignore that--the important thing is that it's free and it works. The program couldn't be much easier to use; just select your series of photos and the program automatically lines them up and stitches them together.
If you like the result, save your finished panorama. You don't get any frills, and the advanced options are what you might call "incomprehensibly geeky." (What value, exactly, should you use for the theta min and theta max?) But it certainly is free.