Collagr is a neat little online app that will turn a bunch of photos into a collage. All you do is give it a URL (on flickr or photobucket) to the photos you want to use & it will mix them into a collage. You can change a few settings like background & colour. From what I can see all creations become publicly viewable.
Here’s one I made from some photos of Canada:
Here are a few examples from some more creative souls: