Hugin is an open source panorama stitching program designed to take in multiple pictures, and output a single panorama. It is useful for creating high resolution panoramic images, or even HDRI environment textures for use in 3D animation. It is available for Windows and Mac.
Hugin WebsiteHugin is not going to win any awards for being intuitive. It will take a little while to figure out how it works, and unless you use it often, it will require some re-learning each time. However, it is very powerful, and can usually produce the desired results.