I'm looking for a good site that lists Hotels and B&B on top of Google Maps or Bing Maps. That has tool tips with prices and availability.
What is the best site of this kind?
Hipmunk.com is exactly what you need. It meshes Google Maps with a list of hotels. You can filter by price, amenities, hotel chains (including B&Bs). You can get heatmaps for food, tourism, shopping, nightlife, and vice.
Runner up: Kayak.com. Similar to Hipmunk, with a more conventional list of options.
Also see these B&B and Hostel sites with map views:
Hostelworld maps all available accomondations on a GoogleMap. Maybe this is something for you. It is more a online booking tool for hostels, but you can also find pension, b&b, appartments and cheap hotels.
Ebookers also maps the available accomondations on Google Map. There you can find all kinds of hotels and even flights, cars, etc.
And another big site, Booking.com, lists all available accomondations on Google Map.
I think nowadays almost all more or less serious site have a functionality like this.