The following information was largely copied from this link, but with some updates. Let us know of any corrections or updates.

Global

• Open Street Map
• Open cycle Map (includes contours)

Specific countries or regions:

• USA
• Svalbard and Mayen
• Iceland
• Norway GodTur with altitude profile function and more
• Norway with indication of types of mountain huts and DNT-huts
• Norway Norgeskart
• Norway FINN kart with layer of aerial photographs
• Sweden
• Sweden Länskartor choose the best screen resolution and then search by province and municipality
• Swedish Fjällkartan
• Finland Retkikartta
• Finland
• Estonia
• Lithuania
• UK with login to access 25k topographic maps
• UK
• UK where’s the path?
• UK Ordnance Survey maps on Bing map
• Ireland with a very basic topographic map layer
• Netherlands
• Belgium>
• Grand Duchy of Luxembourg
• France, Corsica and French overseas isles by Géoportail has an aerial photograph layer and whole Switzerland is also covered by a separate topographic map layer
• Germany, Switzerland, Austria and part of the Italian Alps by Outdooractive, has also a huge database of possible hikes, bike tours, ski tours,…
• Germany with first class information about long distance trails
• Bavaria
• Switzerland
• Austria
• Poland
• Czech Republic
• Czech Republic> alternative option
• Republic yet another option
• Slovakia>
• Slovenia
• Slovenia other option
• Croatia with layer of aerial photographs
• Romania supplemented by a few trails in Google maps
• Russia with global coverage in satellite images and a basic map, especially interesting for the unique geotagged photo collection over Russia
• Portugal and isles
• Algarve
• Spain
• Spain other option
• Galicia
• Basque country
• Catalonia
• Andorra
• Italy
• Peter’s Maps has several interactive maps for some European countries
• New Zealand
• South-Africa
• This Czech list contains many topographic maps from various corners over the world.
• Topographic map list at University of Texas
• Wanderreitkarte is a source for hiking trails and long distance trails in some European countries.
• ArcGIS Explorer is an online GIS application by Esri where you can find a wide variety of maps from a community of users. By registering you can even make and edit your own maps and share them with the world if you like. For a complete understanding of the application and how to browse the maps, use the help function. The entire USA and New Zealand are covered in detailed topographic maps. To find them, type in the search bar in the right upper corner one of the following names to find one specific map: