This isn't four, but I just found this link, and it's
SO cool!
http://www.jonasso...&lon=-99.84375&z=3 It allows you to view Bing maps and Google maps simultaneously. This is amazing to me because I find the imagery from two distinct companies offers a huge increase in the amount of information one can obtain using satellite imagery.
The only big downfall that I can tell for this map is that Bing's most useful feature (IMO), Bird's Eye View, is not available for the purpose of this web sites.
I am slowly finding out that there are a great number of tools and add-ons for these two maps, especially Google and Google earth. If anyone else knows of any useful/interesting/entertaining examples, please add them to this thread!