1 API To Rule All Maps: Mapstraction
The title is fun, I know (and I love Lords Of The Rings).
Mapstraction has kind of a similar logic. It provides a common API for the major mapping providers. This means, when you code with Mapstraction API, you can easily switch from Yahoo Maps to Google maps or else.
You can even use this power to create creative solutions like this syncronizing maps example:
Some features of this mapping API:
Compatibility: All Major Browsers