Here is a Google Earth KML network link that displays a variety of road maps, terrain maps and alternative satellite image sets in the area you are currently viewing. The most of these maps are based on OpenStreetMap, but there also layers for Google Maps, Bing Maps and Mapquest Open.
This allows you to view your favourite map together with other Google Earth layers and in 3D:

To get started, simply download the file
map-overlays-for-google-earth-combined.kmz (version 3) and open it in Google Earth. You can switch between maps by clicking on them in the side bar:

By default, maps will be displayed with an opacity of 100%. In order to see more of the satellite imagery base layer, you can change the transparency of a map using the slider at the bottom of the "Places" section of the side bar:

Whenever you change the view in Google Earth, the image overlays are automatically updated. Zoom levels for map tiles are calculated independently for different regions in the Google Earth window, based on distance to the virtual camera and on latitude. Consequently, maps are updated very quickly and the number of map tiles loaded per update is more or less constant, even if the view in Google Earth is tilted.

Currently, 30 different map sources are supported.
Download the file
map-overlays-for-google-earth-combined.kmz (version 3) for network links to all of them.
Alternatively, separate KML network links for groups of maps or even individual map sources can be found here:
OpenStreetMap
Google Maps
Bing Maps
Outdooractive Maps
MapQuest Open
World Maps
Regional Maps
Cloudmade
OpenStreetMap Mapnik (
http://www.openstreetmap.org/ ):

OpenStreetMap Mapnik B&W (
http://www.openstreetmap.org/ ):

OpenStreetMap Osmarender (
http://www.openstreetmap.org/ ):
OpenCycleMap (
http://opencyclemap.org/ ):

OpenStreetMap Hike & Bike Map (
http://hikebikemap.de/ ):
OpenStreetMap Public Transport (
http://www.öpnvkarte.de/ ):

Google Maps Road (
http://maps.google.com/ ):

Google Maps Terrain (
http://maps.google.com/?t=p ):

Google Maps China (
http://ditu.google.cn/ ):

Google Maps Korea (
http://maps.google.co.kr/ ):

Bing Maps Road (
http://www.bing.com/maps/ ):

Bing Maps Aerial (
http://www.bing.com/maps/ ):

Bing Maps Hybrid (
http://www.bing.com/maps/ ):

Bing Maps China (
http://cn.bing.com/ditu/ ):

Outdooractive Maps Germany (
http://www.outdooractive.com/de/touren/ ):

Outdooractive Maps Austria (
http://www.outdooractive.com/de/touren/ ):

Outdooractive Maps South Tyrol (
http://www.outdooractive.com/de/touren/ ):

MapQuest Open Mapnik (
http://open.mapquest.com/ ):

MapQuest Open Aerial (
http://open.mapquest.com/ ):

Maps-For-Free Relief (
http://www.maps-for-free.com/ ):

Nasa Blue Marble (
http://mike.teczno.com/notes/blue-marble-tiles.html ):

OSM Terrain Layer US (
http://mike.teczno.com/notes/osm-us-terrain-layer/background.html ):

Freemap Slovakia Hiking (
http://www.freemap.sk/#m=T ):
Cloudmade The Original (
http://maps.cloudmade.com/?styleId=1 ):

Cloudmade Fine Line (
http://maps.cloudmade.com/?styleId=2 ):

Cloudmade Tourist (
http://maps.cloudmade.com/?styleId=7 ):

Cloudmade Red Alert (
http://maps.cloudmade.com/?styleId=8 ):

Cloudmade Fresh (
http://maps.cloudmade.com/?styleId=997 ):

Cloudmade Pale Dawn (
http://maps.cloudmade.com/?styleId=998 ):

Cloudmade Midnight Commander (
http://maps.cloudmade.com/?styleId=999 ):

Do you have comments, bug reports, or suggestions for other map sources? Please leave them below.