I have not used it myself, a friend of mine told me about it, but I do not know which one she uses.
I can ask her, but it will be tomorrow, she goes to bed really early.
If you want to peek at some yourself in the mean time, here are some links I pulled up real quick.....
Shape Collage - Automatic Photo Collage Maker
Picture Collage Maker - Digital Photo Collage Software - Download Free
Free Picture Collage Maker
Picture Collage Maker Free - Free software downloads and software reviews - CNET Download.com