Autostitch Review
Autostitch is a free panoramic image stitcher. It takes your groups of pictures and merges them into a single cohesive panoramic image. Unlike most panoramic programs, the pictures don't have to be in any special order, and they can be vertical and horizontal mixed. This alone is reason to try it in my opinion.
The program is very simple to use. Just set your options in the options menu, and click open to pick the group of images that you want to stitch together. By default, the program limits the memory usage and the size of the image, I'd recommend increasing the amount of memory used, and the size of the image in the options, since the default settings are pretty bad. From there, the program will start processing the images, and when it's finished it outputs them to your default image viewer.
The program is fairly stable, however, it has a tendency to run out of memory on larger panoramics with many images. The best way to avoid that issue is to increase the amount of memory used in the options menu, or dropping the output resolution a bit.
This program is highly recommended. It's much easier to use, and much more powerful than the panoramic programs that come with most cameras. You can download it here