On my (old) Windows machine, I used the functionally excellent but a bit clunky to use Mazaika to create some very nice mosaics.
Mazaika isn't available on the Mac, but there's another, free, mosaic builder, MacOSaiX. It doesn't have all the knobs and tweaks Mazaika has, but it does allow for directly searching through Flickr, Google image search or Yahoo! image search streams. Something I only could achieve in a very roundabout way, using Mazaika.