I was browsing around Europeana, which made international news with its debut and subsequent crash in November, when I found that it is a great resource for archived historical maps. If you visit the site and search for, say, "map", you'll get all sorts of results. The "refine your search" tools on the left are good for filtering by country, provider, or other parameters. For instance, I filtered on Bibliotheque Nationale de France as the provider and found a great historical map of the British colonies in the USA and southwestern Ontario. Clicking on the "view in original context" will take you directly to the source, which in this case takes you to a full-screen viewing application when you double-click on the map.
The British Library, another "provider" search refinement option, has an impressive number of historical maps as well. They are also available in a nice flash viewer, such as this 1530 map of Ireland.