In the past few years some excellent sources for old and interesting maps in the UK have been added to the Internet.
The National Library of Scotland Georeferenced Map Explorer is amazing. It has OS maps from the 1873 25 inch to the mile onwards to 1970 1:10,560. It also has several type of LIDAR images.
Historic England has now mounted much of their historic monument catalogue online as a Map Explorer. This even includes some filled in bomb craters.Tithe maps
The National Library of Wales has put tithe maps for most of Wales online.
In England some tithe maps are available online from local authorities.
For others search Google, although a lot of counties have farmed out their digitised maps to commercial sites such as the Genealogist. Also see below for county viewers.
Other County Maps
Dorset has an advanced county map online with infra red aerial photography amongst many other features.
Cornwall has a map viewer for the county that includes historic mines etc.
Know Your Place West of England has provided the following historical map viewers that go down to tithe map level and include 1946 aerial photography:
National Collection of Aerial Photography although many images are not yet digitised.Tweet