DipStych is an Open Source program that will surely be useful to you at one point or another. It will essentially take multiple images and put them side-by-side or on top of each other. Sure you could copy and paste images into other programs but when you are trying to bring more than two images into one single file it could get tedious.
This is useful for me because I can easily put screenshots together for tutorials to show what to do in step 1, step 2, and so on. It will generate one nice flowing image for me and you can even decide whether you want to put a border around the images.
There is a Flickr homepage for DipStych that discusses some of the cool things that it does but you should really checkout the DipStych tag on Flickr because it shows some of the photos that other people have used DipStych for.
Believe it or not this is one of the few programs that I have found to merge multiple images. I have seen some before that will do two images or even a few applications that can handle multiple inputs but they aren’t free. DipStych is unique in that sense because it is both free and can handle multiple photos.