I think that mosaics are one of the most striking effects you can apply to photos and few do it more easily than MozoDojo.
MozoDojo is a photo-mosaic creator that handle resolutions suitable to blow-up into a poster. It's extremely simple to use - just drag and drop an image into the main interface, or browse to it from the File menu. It's supposed to work with iPhoto and iTunes too and although I found it was integrated with them, I couldn't actually select any photos in my folders. It also lacks Picasa support.
MozoDojo is a deceptively powerful application, requiring the combination of CoreImage, QuartzComposer, CoreData, Spotlight, Quicktime and multi-threading - and yet it still remains very fast. Problems can occur though when saving your final images. Sometimes MozoDojo may hang or just crash on saving, although the majority of times it works fine.
Quick and fun to use, MozoDojo creates excellent Mosaics, although it can be prone to more than the odd bug.