Great news on the other side of the Atlantic for Google Maps type mashups: Ordnance Survey, the British USGS (with some remarkable differences, like free access to map data) is making available (for a restricted number of beta users at this point) their OpenSpace API, according to Silicon.com:
"...it will only be available for non-commercial use so that it doesn't compromise the OS' relationship with existing commercial partners, which pay for mapping information."Some more info here. And Ed Parsons posted about it too. [Update: Also covered by National Geographic.]
And talking open maps, check MapGuide Open Source and OpenStreetMap API and its usage of TIGER data.