Our pals over at Curbed have been pretty good at rounding up all the Real Estate mashups-- yesterday that pointed out the Real Estate Advisor Map and Natefind. And then there is the granddaddy of this type of map: HousingMaps (select NYC in the dropdown), with results from Craigslist. There's also Oodle Housing-- which gloms from a bunch of services. Maybe someone should do a meta-meta-mashup of all of these maps! That would be a lot of meta!