Check out our new site Cropdog.com. You can create many collages and mosaics in 12x12, 8x8, 8.5x11 and other sizes. Plus there is a special size to create a FREE Facebook Banner (shown above). There are many designs to choose from and it is super easy. Just load up your photos and insert them into the spot you would like to place them.
Here is a little tutorial on how to use Cropdog: