Hugin, powerful Panorama Tools

Open Source is the program for creating panoramic photos, including free ones, I gave the best results. Has many more options than Microsoft ICE, however, has a limit that will not please many: it is semi-automatic and fully automatic.
In fact, for best results, requires some little user intervention (for example choose between photos of the panoramic one down as a marker for exposure, etc.).
If you want to spend a few minutes to learn how to use it, will give some great satisfaction. If you want something that works the first time around then the best choice is Microsoft ICE

Hugin