It's a replacement for the stock Car Home, and it's free on the Market. It's listed as Car Dock with a version number after it. It was at v2.1 when I opened this thread, and as of my last edit, it's listed as simply Car Dock v2, and it's by IG88 Apps.
There was some discussion of this in another subforum, but not in the Applications subforum.
It gives you 4 home screens that are fully-customizable with 6 apps/shortcuts per screen. Because it upscales the low-res icons, it doesn't look very good at first.
However, if you go into the Help (by pressing the Menu button), it'll tell you how you can use completely custom high-res icons that you put on the SD card.
For those who prefer nothing happens when putting their Droid in the Car Dock, try Dock Nothingness ... also free on the Market.