Mapstraction
 | This project seems to be unmaintained and dead. |
Mapstraction (mapstraction.com) is an open source JavaScript library for Deploying your own Slippy Map. It is an abstraction library wrapping various mapping libraries. This potentially allows a web developer to learn one API, and then easily swap between different JavaScript Slippy Map implementations, without re-coding anything.
Note that swapping between different tilesets (or presenting multiple tilesets of layers) is a pretty standard feature of any JavaScript maps, so you can for example present OpenStreetMap tiles in using the Google Maps API, despite it being normally used to display Google Maps tiles. Mapstraction offers a different kind of flexibility. You can hot-swap from one API to another, such as from Google Maps to OpenLayers.
Development
The project is open source (BSD license) and code is located on GitHub.