Here is a good explanation of the Quad-Key system used by both Bing and Google maps. http://msdn.microsoft.com/en-us/library/bb259689.aspx

Similar, but without sacrificing accuracy for built-in zoom information. Due to it's layout, they also support using bit -wise operators for all of their operations (since in reality these systems are all just interleaved x/y bits)

Thanks for the link. I was familiar with google's system from when I worked there but couldn't find a good description of it so I assumed it was purely internal.

The S2 library is here: https://code.google.com/p/s2-geometry-library/

It's based on the hilbert curve.