Pixable has a great deal going on right now for a photo mosaic. I ordered a 11x17 print for under $6 (including shipping)! Thanks Carrie from Pocket Your Dollars for pointing out this great deal! (go to her site and search "pixable" for the code)
This was a perfect way to use 100 photos from all of our visits to the arboretum over the years. Every single picture in the mosaic was taken at some point in the past three years we've been taking spring portraits by the crab apple trees. I think making a mosaic from a bunch of vacation photos would be fun too, but we never go on vacation so that won't work for us - but maybe for you? Let me know if you make a mosaic - I'd love to see it!