Panorama tools is a high quality and free panorama stitching software package. Notably it contains pano12 and PTOptimizer, both required by Hugin.
Hugin provides an easy to use graphical user interface for Panorama tools.
Available for most Linux and Unix varieties (plus OS X and Windows)
autopano-sift: Automatically identifies control points. Works with Hugin
PTStitcher: The original Panorama Tools rendering engine, now superceded by nona
nona: An open source replacement for PTStitcher. Part of the Hugin package
Enblend: Overlays images so to make the seam invisible. Works with Hugin
ImageMagick: Optionally batch process images before using Enblend, e.g. when you have large black areas because of a pitch angle.
PanoCube: Converts equirectangular (360x180) panos to QuickTime cubic movies.
Quicktime can be installed via wine
Panorama Tools Plugins
clens command-line version of PTLens
PanoPoints: Perl-Based control point picker for use with Panorama Tools