"Envisioning Development: What is Affordable Housing?" is an interactive Flash-based map of housing and income statistics on a neighborhood-by-neighborhood basis throughout the New York City area. Although I've seen similar statistical/analytical interactive maps with similar data plenty of times, the aesthetic design and information visualization "impact" of this map is almost breathtaking. This map is part of an online resource toolkit for urban planning and land use education based around NYC, offered by the Center for Urban Pedagogy.
The California Inclusionary Housing Policy Database provides a geographic search capability for users seeking information about policies in specific regions or municipalities in California. The database, possibly the first of its kind in the US, is managed by the California Coalition for Rural Housing, and was developed by MT development partner CTC in 2008-09 thanks to funding from the S. H. Cowell Foundation. The database and associated website are built on the Drupal content management system, using GIS functionality supplied by Google and CTC's mapping software.