As a fan of old maps and knowing how useful they are in family history research I thought I would recommend Oldmapsonline.org.
The website offers an easy to use gateway to the map collections of many libraries and archives across the world. It provides a search engine based either on a Place Name Search or by Map Navigation. Quite often it is faster and easier than the search engine of the organisation holding the maps. At the moment 383804 maps are catalogued and accessible through this gateway. The best coverage is for Britain, Europe and North America.
In the UK two of the most useful old maps websites, the National Library of Scotland and British Library, can be searched through this gateway.
For more information on maps and your family tree see our article on Maps