Google Earth and all the similar programs are great, but I like the good, "old" paper maps, which you can sit and look at in a cozy chair and of course bring with you to the areas where you really need them.
On this page I hope to get a good link list together.
If you know any good sources, please drop me a line and I add the link.
Free maps, for downloading or viewing online
Map Server. The site is a goldmine, filled to the brim with scans of excellent maps from various sources. The site can be a little difficult to navigate.
Most of the maps show areas which are not so easy to find elsewhere. If you look for maps of the Rockies or the Alps, you'll find nothing of interest here.
Here's another goldmine! Poehali.org or Let's Go! offers 32 000 maps from all over the world. Many of them are formerly hard to get Russian topo maps. A very useful resource. It takes some time to find the right area and the downloading can be slow.
Their sister site - Mapstor offers all the same maps, but there you have to buy them. They come at an affordably price. The below offer is hard to beat:
29 maps of scale 1:500 000
183 maps of scale 1:200 000
22 maps of scale 1:100 000
Price $19. See here.
Blank on the map has a very good selection of maps and info for Karakoram, Hindukush, Hinduraj and Himalaya.
Prames Mountaineering site with maps and online viewing. For Spain, mostly in Spanish, but some parts in English.
Japan Two websites of topographic maps of Japan in Japanese. They use the same data but offer different interfaces: CyberJapan has all scales from total down to 1:2500. Watchizu shows only the topographic maps.
Switzerland. More Swiss maps on Mapplus. In English.