Map comparison sites always seem to be very popular. One of the most popular searches on Google Maps Mania seems to be for sites that let you view the different online map providers side-by-side.
Buncha Maps is a new application that lets you view Google Maps, Bing Maps, ESRI Maps and OSM all in the same window. Actions in all four maps are synchronised, so if you pan and zoom in any of the maps the other maps will also show the selected view.
Dual Maps is a map creation tool that allows anybody to make an embeddable map that includes a Google Map, a Google Maps Street View and a Virtual Earth Bird's Eye view of the same location. You can choose between the various options of either map service, which means you can have the Google satellite view alongside Virtual Earth's Bird's Eye view or the satellite/aerial View in both maps etc..
Bird's Eye View
This map places Bing Maps' Bird's Eye View side by side with Google Maps' Aerial View. You can view a number of locations with both map providers' oblique views.
Maps Compare is a website with four different on-line map services on one page. The site places Google Maps, the Google Earth browser plugin, Yahoo Maps and Bing Maps beside each other, which can be quite useful if you want to compare the map coverage provided by each provider in different locations.
Where's the Path
Where's the Path displays Google Maps alongside the corresponding Ordnance Survey Maps.
GeoTrubu also has a comparison of Google Maps and OpenStreetMap and also shows Yahoo Maps and Bing Maps. The site allows you to select the base layer for each map.