Via Sociological Images, these maps show how much of the Earth has been colonized.
There's a map showing every road in the world, and every railroad, and all the navigable rivers. The road one is particularly cool, because the parts of the world that aren't settled stand out really starkly -- mostly deserts, but you can also see the Amazon.
Then they pull all together:
Considering all of these modes of transportation and the type of terrain, the New Scientist calculated how long it would take, from everywhere on the planet, to get to a city of 50,000 people or more [by land or sea]... They estimate that less than 10% of the world is more than two days from the nearest city. The most remote place, they calculate, is the Tibetan plateau.
The full-size maps are on the New Scientist site and are very worth looking at. You can also see maps of other things, like shipping lanes, and some interesting detail blow-ups.