Hugin is a panoramic imaging toolchain and easy-to-use GUI frontend, based on the Panorama Tools library. It allows you to assemble a mosaic of photographs into a complete immersive panorama, stitch any series of overlapping pictures, correct for various lens characteristics, work with high dynamic range images, automatically identify control points for image matching, and much more. Tutorials: http://hugin.sourceforge.net/tutorials/
WWW: http://hugin.sourceforge.net/