For years, I've used Canon's PhotoStitch app, which came with a printer I'd bought long ago, to create panoramas and stitch pictures together. Despite the increasing age of PhotoStitch, it has served relatively well.
Relatively well, that is, until recently when I was scanning some sheet music for my roommate's father. The music was written on large pieces of paper and I had to scan each document in sections. PhotoStitch got hopelessly confused trying to deal with all the horizontal lines on the page. A quick search lead me to DoubleTake, which I installed and immediately, I was able to combine the sections of each scan to make a perfect picture.
Since then, I've used it to redo some older panoramas and so forth I had made, and to finish some that I'd never been able to complete due to PhotoStitch being unable to handle it. The software has performed flawlessly in nearly all of my attempts, and gives you a wealth of control over how each image is processed without making it seem overly complicated.
This software is entirely worth the $25, and then some, for what it does.