Maps, Inc.
Maps of all sorts -- a really great resource for maps. International online maps are kind of weak but if you want to buy any type of map (street, atlas, topographical, trail, etc.), this is the place to go.

MapQuest
Absolutely great for local maps & driving directions in the US & Canada. They claim to have good international maps and they do have some general ones, but for other countries you'll want to try another source.

MultiMap
The UK is this site's strength but also very good for English versions of maps & directions in all of Europe. Some pretty decent maps for many other countries as well.