The new Google street side view layer in Google Maps
Of course this isn't new, after all Amazon A9 did this back in 2005, but neither was online mapping applications when Google Maps launched. The question isn't whether they're were first, it's whether their interface is so much better than what's gone before (yet again) that they corner the market. That, and from the sounds of things, whether this gets integrated into Google Earth.
However while the guys over at O'Reilly seem to be able to see this on the Google Maps pages right now, it looks like the rest of us will have to wait for the official announcement during Where 2.0 later today. I certainly can't see the new features and from the comments on their post it doesn't look like I'm alone on that one.
Update: I can confirm that adding
&gl=us(via Lifehacker) to the URL does indeed allow you to use the new features, at least from the UK, and from what I remember it blows the interface for the long defunct A9 service out of the water. The way you can click and drag around your location is actually pretty slick.
Update: The guys over at the Google Lat Long blog have pointed us to a video showing how to use the new features.