They use OpenStreetMap‘s data, which you can help map the world: get your own GPS system and you’re on the go. [You can also use OpenStreetMap's APIs but it is not as user friendly to deal with their API and the look and feel is not so great, no matter what style you apply.]

You can use CloudMade’s API to build your own maps. That’s how I discovered them. Their API is really easy to use and if you have questions on the integration, they’ll help you right away.

UI wise I love the look of their maps. It looks way better than some others’…

Here is their mission statement:

CloudMade help you make the most of map data. We source our maps from OpenStreetMap, the community mapping project which is making a free map of the world. Our aims are to continue the democratization of geo data and to expand access to open geo data through a range of simple yet powerful tools and APIs.

Go Cloudmade and OpenStreetMap.

Snapshot of Cloudmade's look & feel