Enblend
overlays images so to make the seam invisible.
Works as a plug-in for PTGUI. Works also standalone.
For Windows and Linux. Mac OSX version (xblend) is available from Kevin Kratzke
If you are working with multi-row panoramics, (for example three rows of eight pictures each), you can greatly speed up the processing by manually feeding the first row of images (with the -a parameter which assembles non-adjacent images first), then the second row of images, and finally the third row of images. Next, feed these three assembled rows into Enblend again using the -a parameter and you will have a fully blended pano in a fraction of the time.
I have written batch files for the Windows platform that do this automatically. Feel free to email me at markdfink@northernlight.net if you are interested.
Also, other parameters worth noting are: -w to wrap the blending process around the 360 degree boundary so you don't end up with a harsh transition at the +180 and -180 degree seam. -o to force Enblend to use an output filename of your choice -v to see the details of what is happening rather than staring at a blank screen