|NOT free, but still pretty much the most capable AFAIK is PTGUI. I use the "pro" version. PTGUI began as a GUI for Helmut Dersch's PanoTools, and thus the name. You can still use the PanoTools stitching engine but you don't need it.
PTGUI has two modes, the simpler mode pretty much does the heavy lifting for you. If you're a control freak (like me) the Advanced mode provides all the options you could ever want.
Even in the simpler of the two modes, this is not a program without a learning curve. But the results are well worth it. If you stitch large panos, and if you want to control and tweak stitching to get results that match to within a pixel, it's hard to beat PTGUI. I've tried others, and always come back.