It's a web site: http://flagrantdisregard.com/flickr/mosaic.php
You can use your Flick! albums as the source for the images, or you can upload from your hard drive. You pick a type of layout (you can even do trading cards, movie posters, magazine covers, etc.) and upload the photos and it builds the pages.
Then you follow a link to upload the result to Flickr! but on the page after the upload link you can bail out and save a copy to your disc, then edit it, etc., and then you can upload that to your Flickr! album (or not, your choice).
Flickr! itself is free, but you must register. You can upload up to 2 gigabytes of photos per month and store them in up to 3 different photo "sets" in your own album. You can make them individually public or private and set various levels of permissions of use. Or you can buy a Pro account (not expensive) and get much more storage and a practically unlimited number of "sets" -- which are like folders and Very convenient for arranging images.
Recommended -- especially if you don't have your own web site for this sort of thing.
Here's my Solar System album:

and corresponding link here.
And here is my Mars 2005 Apparition "album cover":

and corresponding link here.