The Geospatial Portal Beta is built completely on Open Source Software. From the site:
It utilizes the OpenLayers mapping library.
It uses Tilecache to serve the maps (WMS), which in turn uses GeoServer as the back end (which in turn uses PostgreSQL/PostGIS).
All tabular data is pulled from REST web services (JSON) dynamically via AJAX, including spatial analysis via PostGIS. No page refreshes.