"Inkarnate Worlds is a web-based cartography tool that allows you to quickly bring your fantasy worlds to life."
The software is still in Beta, and therefore very basic. It has a very distinct look, and not many options for customanisation. It is easy to learn, and is good for the very first sketches of your world, but will never have the refined look of some of the other software.

The planet-map generator shown of this page has been developed in small increments over more than 20 years, so the code is a bit messy. But if you just want to run the program, this shouldn't matter for you. As of Febrary 2013, a web interface is available. It does not have all the options of the offline version, and (so as to not overburden the server, which is shared with many others) the size of maps is limited. But it is easier to use and does not require any installation. The offline program runs from a command line, so no fancy GUI. Lots of command-line options.
This image was made with the online version.

First release of GenaMap is now available for windows (sorry mac/linux users)! Please note this is an alpha, so don't be surprised if you run into some bugs, crashes or weird behavior.

GenaMap is a tool for worldbuilders to assist in the creation of maps, which can be a long and tedious process. GenaMap can, within seconds, generate realistic maps with many customizable options and features in order to create a map tailored for your needs.

"Make your own custom Map of the World, Europe, the Americas, United States, UK and more with colors and descriptions of your choice"

This site let's you color in maps of the world, and it is good for creating an alternate universe.

"Hexographer was built from the ground up to let you make maps in a few specific styles quickly. By eliminating a large number of cumbersome options general image editors & computer-aided-design (CAD) programs use, Hexographer lets you make a map in a tenth of the time it takes just to learn another tool."