It's called Expression Web SuperPreview.
It looks very promising. Going to try it out tomorrow.
Update: Meh! The app was more or less useless compared to running browsers in Virtual Machines. The app lets you preview IE6 + your native IE version. If you have IE6 installed (which I have due to our customers lack of will to upgrade), you can only preview in IE6.
The preview is more or less an image. You can't click links/navigate to different parts of your website. To navigate, you have to type in the address. I guess it's a step up from browsershots.org, but that's more or less it.