The New York Times looks at mash-ups, focusing on Platial. (I am very annoyed with the New York Times at the moment, BTW.) Having seen a lot of these mash-ups, Platial looks oddly old-school to me. In terms of features, Tagzania is ahead… and map-neutral to boot — see tagged places in just about any map, as well as Google Earth.
(Wired also wrote up Platial a few days ago.)