From the blog post Virtual Earth Team Launches Street-Side Drive-by we learn

The Virtual Earth team is pleased to launch a preview of a new feature we have been working on – interactive Street-side browsing. You can try it out at Street-side imagery allows you to drive around a city looking at the world around you as if you were in a car. But unlike the real world, you can stop your car anywhere you like and rotate your view around 360degrees. Currently we have street-side imagery for San Francisco and Seattle online, and we are planning to have many more cities added soon.

One of the most interesting features is to put yourself in ‘Street’ view map style. In this mode, all of the street-side images are pasted flat on the map to give you a very unique overview of an area. It takes some getting used to, but once you adjust to it you’ll find it provides a very compelling companion view for our Hybrid maps. Street view helps you orient yourself quickly in an area, while the street side views then show more detail presented as you would see it in the real-world.

This technology preview is just that – a means for us to get a feature we are working on in your hands to play with and provide feedback on, before it is ready for prime time integration into the Windows Live Local site.