I've always wanted to be able to create one of these, and finally there's a tool that can make it happen. Although it's not free, it's worth posting about because of how well it works.
That photo of me above was made by PrintMosaic, an app that generates a large photo mosaic out of smaller pictures. You start by uploading a big picture and then a bunch of smaller ones. (If you happen to have your photos on Facebook, Dropbox, or Instagram, you can connect and grab them directly from there instead.)
You can choose from four different sizes. Once your mosaic is generated, you can see an interactive preview of it. Hovering will enlarge the smaller images:
While they recommend using a minimum of 100 small images, I only used 53 from my Flickr gallery and the result was still very good. Alas, the service isn't free, but if you share your mosaic online you can save $5 off the $6 cost of a download. There's also an option to have it printed on a poster for $6 and up.
Do you know of any other services that let you create photo mosaics?