Worldmapper: The world as you've never seen it before
The WorldMapper site shows a wide range of social and demographic data, including a whole series of tourism and travel related maps.
The example below the relative number of international tourists that each country received in 2003. If you click on the title above, you will go to this map. Click on next and you will go to many more tourism-related maps.
The maps are really cool. And this one also shows a particular pet peeve of mine. It is obvious that the countries of Europe receive more international tourists than any of the others. Only the US and China outside of Europe have any significant size comparisons. However, are the borders within Europe really international borders? Personally, I do not think so. Passing from one country to another in Europe is no different from passing from one state to another in the US or Australia, or from one province to another in Canada or China. As such, the international arrival numbers are considerably over-inflated in Europe, in comparison to the rest of the world.
Anyway, these are the numbers that we have to live with today thanks to the World Tourism Organization (aka UNWTO) -- a bogus United Nations affiliated organization that charges for most of the data it collects and the services it provides to developing countries -- unlike other true UN bodies.
NEW: Another great source of Thematic Maps (as well as traditional political, street and physical maps) is National Geographic's Map Machine.