I have a soft spot for mosaics that are integrated into a larger LEGO creation.
Milan Bikics' cathedral does this beautifully. It is very eye-catching, and one of its most striking features is the intricately mosaicked roof.
Some other good shots of the beautiful roof can be found here and here , and throughout the majority of his photostream. Last I heard, the cathedral is still a work in progress. Once Milan said that his goal was to acquire one of every kind of LEGO piece ever made and to use them all in his cathedral. While I tend to focus on a limited palette of pieces, I think the "more-the-merrier" technique is quite effective here.
(I am fairly confident that Casey hasn't posted about this already, but if he has, I will just say that it certainly deserves a second look.)