From http://en.wikipedia.org/wiki/AutoStitch
"Autostitch uses methods known as SIFT and RANSAC. This program differs from some other image stitching software in that it automatically stitches together even unaligned or zoomed photographs seamlessly without user input, whereas others often require the user to highlight matching areas for the photographs to merge properly. The only requirement is that all photographs be taken from a single point."
Free demo for Windows (which works seamlessly on Linux under Wine) is available, and I never needed another tool for stitching of pics.
As mattdm correctly wrote above (in comment), this is similar question: Which tools are good for creating panoramas/stitching multiple photos?Which tools are good for creating panoramas/stitching multiple photos? and autostitch is also there one of the answers. I love (and prefer) it for two reasons:
- simple installation (one exe file, nothing to install)
- simple to use - choose input pics, choose size of output image, and other settings you do not need to set/change if you don't want to
So it's not exactly open source, but it is free (demo), simple and works fine both on Windows and Linux.