If you have Irish ancestors then Griffith's Valuation might be able to help you search for an address or a particular person. This site from Ask About Ireland plots the Griffith's Valuation on Google Maps.
Griffith's Valuation was the first full-scale valuation of property in Ireland. It was published between 1847 and 1864 and is one of the most important genealogy sources surviving from 19th century Ireland.
To search the Griffith's Valuation you can enter an ancestor's name or a place. The results of your search are then displayed on a Google Map and in list form.
World Family Names
If you have nothing to go on but your surname, when searching your family tree, then you should start with Public Profiler's World Family Names.
If you enter a surname into World Family Names you are presented with a heat map of the world showing where there are high concentrations of people with that name.
Beneath the map the top countries, regions and cities where your name occur are displayed in list form.
England Jurisdictions 1851
Genealogy website Family Search has created a nice Google Map that lets you search for places by county, parish, hundred and province in England, as they were constituted in 1851. The map should prove to be of great help for anyone who wants to search the geographical origins of ancestors from England in the 19th Century.
You can select to view the different administrative boundaries via a drop-down menu. You can also select an old Ordnance Survey map as the base layer instead of the Google Map. You can search for towns by name and it is also possible to click on the different administrative boundaries on the map to view further details about a location.
Although there are no Google Maps on FamilySearch, we should give this website from the The Church of Jesus Christ of Latter-day Saints a mention. FamilySearch is the largest genealogy organisation in the world and is probably the best resource on the Internet to start researching your family tree.