Ten Things About Worldbuilding
Freeware caution: always scan free downloads of anything for bugs and other threats before dumping the programs into your hard drive.
Want to see a worldbuilder at work? Stop by Are you a God?, Nils Jeppe's art, writing and worldbuilding blog.
AutoREALM is "a Free GNU mapping software (a "cartographer") that can design maps of castles, cities, dungeons and more. AutoREALM is generally used by Role-playing Game practicants who enjoy doing their own maps. But it could fits the needs of other people. If you are a Role-Playing gamer or else, you are cordially invited to join the AutoREALM community: fellows gathered around a free hobbyist map tool. Originally made by Andrew Gryc (say "grits"), AutoREALM is now Open Source, creating a unique opportunity for the RPG world to mix graphics and computer programming" (OS: unspecified; also I couldn't access the download link pages on the official freeware site but I found a download link over on sourceforge.net here.)
If you need an interactive app to generate and view random fractal planets and terrain with oceans, mountains, icecaps and rivers, check out the Fractal Planet and Terrain Generator (OS: unspecified)
If you're interested in building a language, you might try the tools and advice available in Zompist.com's The Language Construction Kit.
One of my more recent finds among online sources about worldbuilding, The Mythopoet's Manual ~ Writing Multi-Cultural Fictional Settings by Loren J. Miller, is simply fabulous.
Terragen Classic is "a scenery generator, created with the goal of generating photorealistic landscape images and animations. Terragen is free for personal, noncommercial use, with only a few limitations. Terragen Classic is capable of near-photorealistic results for professional landscape visualisation, special effects, art and recreation. Terragen Classic has been used in a variety of commercial applications including film, television and music videos, games and multimedia, books, magazines and print advertisements (OS: Windows, Mac)
Planet Engine is "a 3D real-time planet renderer. It allows you to fly over the surface of planets and also to see them from space. You can explore real planets - like Mars and Earth - or use imaginary data" (OS: Windows)
One of the best matter-of-fact articles on worldbuilding that I've read in a while: The Power of Worldbuilding by Steven Savage.
To get some planetary ideas without having to plow through tons of generated stats, take the Random Planet Generator for a spin.
Patricia C. Wrede's Worldbuilder Questions are hosted on a fan site and contain all of her old FIDONET postings. The questions are like writing prompts for creating and fleshing out your fantasy worlds; they cover just about everything a worldbuilder could possibly want to think about and then some.