There is a lot that came out of Where 2.0 and Wherecamp that directly affects this discussion and I’ve been doing my best to digest it all.  Here are some of the highlights:
Google

Flash API for Maps
Feature/Layer Enhancements to Google Maps (Real Estate, Photos, Wikipedia, et al)
Google Friend Connect
REST-based API for GeoSearch

Yahoo
Yahoo Internet Location Platform

Placebase/Pushpin
Free REST-based [...]