FamilySearch.org is on of the bigger genealogy websites with a large repository of digitized and microfilmed documents, a huge collection of pedigrees, and online pedigree management tool among other things. They also have an ongoing FamilySearch Labs program where they try out new technologies and product concepts. One of their current Labs projects is http://maps.familysearch.org.
Historic Maps of England
They currently only have maps of England available and only for 1851. For 1851 you can choose a variety of overlays including the parish, county, poor law union and province boundaries. For some types, such as parish boundaries, you can click a parish and get an info box indicating when records are available for that parish., or click a county and get a list of parishes in that county.
Hope for Future Directions
The current state of http://maps.familysearch.org is a single point in time for a single location. If the project makes it out of labs I hope and expect that they will extend it worldwide and across time. Geocoding the boundaries for their many datasets of genealogical records would be quite a process, but FamilySearch routinely undertakes and is involved in huge digitization projects and does a good job with them.