|England Jurisdictions 1851 (FamilySearch)
If you are interested in something more focussed than maps of the entire world (and beyond - Google Earth goes to the Moon, too), there is plenty to choose from. I think the first online map site I ever saw that compared historic and modern maps was the Charles Booth Online Archive which compares the famous London poverty maps from 1889-1890 with the modern street map. A more recent site which does even more with old and new London maps is London Low Life where you can see 27 historic maps of London, the earliest from 1788. Each of these is overlaid with a modern map, and a slider bar you can use to reveal as much or as little of the old map as you wish. This site also has a thematic map where you can see the changes in the size and density of London's population between 1801 and 1890, and the sites of hospitals, workhouses, prisons and more. It even has its own Victorian version of Street View, in the form of Tallis's Street Views, a series of very detailed engravings of some of London's main streets in 1838-1840. This is an absolutely unmissable site for anyone interested on London history.
London Remembers is a well-constructed site devoted to London's memorials. It is based on a Google Map, which you can search by the names of individuals (real and fictional), events or institutions. There is an intriguing 'Puzzle Corner' feature, where the site's creators are looking for help with identifying or interpreting some of the memorials and illustrations. They have a sense of humour, too. I particularly liked their disclaimer: Caveat: Be aware that London actually has many more cars, fewer bikes, more rain and less sun than our photos show.
London is not the only served by imaginatively designed map sites. Cheshire's e-mapping Victorian Cheshire project contains nearly 500 tithe maps from around 1839-1851, together with their accompanying apportionments. You can search by name of owner, occupier, parish, township and even some field or plot names. You can view the results as a list, which you can download as a spreadsheet, or on a map. The map view shows the tithe map alongside a modern map of the same area, and you can zoom in and out on both of them at the same time. Don't you wish you had ancestors in rural Cheshire? Or if you have, lucky you.
Returning to the wider world, Historypin is a collaborative site where users are encouraged to attach their own historic photographs an stories to a Google map. You need a Google account to participate, which is free to set up, and you don't need to have a Google email to do do this. And returning to the present day, Openstreetmap is an editable street map 'The Free Wiki World Map' as it describes itself. But if you want a site with more features, and that has genealogical applications, you can register with Ancestral Atlas free of charge to share your genealogical data in a map-based format. You can enter details individually, or upload a GEDCOM file, and control how much of your information is made public. You can get access to additional features and an advert-free version of the site for a one-off subscription of £20. Ancestral Atlas links to another of my favourite sites a Vision of Britain through time for extra contextual infromation, and if you haven't explored this site already, then you really should! Another essential interactive map site for English genealogy is of course the FamilySearch England Jurisdictions 1851
Finally, here is an interesting collaborative project at Geograph to photograph every square kilometre of Great Britain and Ireland, which is 78.8% complete as of today - any offers?