The EPA's Radon start page.
The EPA's "A Citizen's Guide to Radon: The Guide to Protecting Yourself and Your Family from Radon"
The EPA's "Home Buyer's and Seller's Guide to Radon"
The National Safety Council addresses Radon, offers testing kits, and outreach materials. [pdf file]
The NSC list of Frequently Asked Questions about Radon (with answers). [.doc file]
The SD DENR maintains this information page about Radon concerns in South Dakota.
The official site of the National Radon Safety Board
The National Environmental Health Association's "National Radon Proficiency Program" - be sure to check the "Checklist for Consumers"
The American Association of Radon Scientists and Technologists publishes this online guide to "Radon Risks and Health Effects"
Lung Cancer: It's Not Just From Smoking! (their slogan says it all)