Today, in a follow-up to their spring launch of Pushpin LE -- a supported, licensed mapping tool with Google Maps API compatibility -- mapping vendor Placebase is taking-on Google Maps again, this time with a Dynamic Layers API. It's the next round in the ongoing one upsmanship of the online mapping feature wars, Placebase now has hooks for multi-layer maps with support for dynamically shaded regions such as market analysis, demographic, and other data. Map tiles are pre-rendered for performance and the API provides the ability to dynamically turn layers off-and-on and to change their z-order. They have an online example here.
In a conjunction with this they're releasing Rendermap, the first service that creates application-specific tilesets for Google style maps (as with Pushpin above). Applications can be hosted by Placebase or by customers with appropriate map servers. To see an example check Fannie Mae Foundation’s DataPlace with 1,300 demographic and housing-related variables.