Street View is the Google Maps extension that lets you view the streets for particular cities in the US just as you would be driving through them. On March 27th 2008 they released the Street View API which basically gives web developers the needed tools to create web applications that interact with Google's Street View service. Almost a month later, there are a dozen of Street View mashups released, some of them being quite nice:
Dual maps let you display for a single location a Google Maps view, along with a Google Street View and a Microsoft Virtual Earth view - all side-by-side and also linked together, so when you move the cursor on one it will dynamically move on the other two.
Povo is a wiki for local places in Boston (for now?). On the wiki page for each place they add businesses, parks, parking lots and any other public information users add. You can search through Povo by location, tags, or for specific questions. Each listing also shows the Street View for that location, one that you can modify if it's not accurate.
RealBird lets you list and find homes for sale, all including detailed information as well as map listing and Street View for the location where the house is from.
A website that lets you search for New York apartment rentals. Has a basic implementation of Street View, so beside showing you information about the rented place (price, location) you can also see its position on Google Maps and have a Street View for that location (where supported).
A website that contains custom world maps, from showing maps for sport events to displaying sightseeing of fun places, Glotter incorporates them all. Now they've added a Street View option that lets you see panoramic street-level views for particular areas. It's also posible to automate the viewing process (rotate, move forward, move backward).
A website that integrates Google Maps and Street View to give you a real look at the Las Vegas Strip.