From time to time, I will collect all the finds listed in this thread and abstract them in this original post for easy reference.
David Rumsey Map Collection
The David Rumsey Historical Map Collection has over 18,460 maps online. The collection focuses on rare 18th and 19th century North American and South American maps and other cartographic materials. Historic maps of the World, Europe, Asia, and Africa are also represented. Collection categories include antique atlas, globe, school geography, maritime chart, state, county, city, pocket, wall, childrens, and manuscript maps.
Perry-Casteneda Library Map Collection
Contains hundreds of links to other historical map websites. The collection is categorised by Continent.
Holy Land Maps - University of Florida
Lots of old maps of Judea/Palestine/Israel arranged in chronological order.
Antique Map Magazine - Mapforum.com
An online Magazine with numerous articles about and examples of old maps.
Genmaps - Old and interesting maps of England, Scotland and Wales
Categorised by Country and County and chronologically.
The National Maritime Museum
Old sea charts from around the world, categorised geographically. Also some continental maps.
An online antique map shop with low resolution images (clickable to higher resolutions) of maps categorised geographically.
Maps of the Past
Another online store with old maps categorised by function, e.g. railroad, civil war, panoramic etc. Mostly maps of the USA but with an international section.
Old maps of cities of the world. Searchable alphabetically.
Reinhold Berg Antique Map shop
Organised by region, maps may be viewed in detail through zoomify.
Free Relief Layers from Google
Uses the google maps engine to show a zoomable relief map of the world.
Solo mapas - Just maps
A collection of about one hundred and fifty maps in varying resolutions. Organised by region.
Great Buildings Collection
Overhead satellite images of famous buildings around the world. Searchable by Building name, architect and place.
Big Map Blog
A collection of lots of old and unusual maps with an in-browser zoom facility so you can see the detail