Onion Map provides a 3D map of many of the large cities across the globe. Unlike Virtual Earth and Google Earth, Onion Map's display is not as realistic and has a very cartoonish look to it (almost looks like you are looking at a SimCity map). In my opinion - this is a good thing - as it differentiates this map service from the other 2 behemoths.
Not all the big cities are available on this website (Denver is sadly not). Such a map is great as an aid for tourists who wish to plan visits to a city as it removes all the clutter that comes with displaying Geo-Specific city models.
Onion map is less of a map service for mapping purposes as it is for research and learning more about a city, as well as using it to get information on businesses. It is almost an encyclopedia on a map.
Looks like it is a Korean company.