• National Geographic   The American magazine covers a huge range of subjects, from space and the environment to animals and even world music (you can listen to thousand of artists). 
• NASA   Videos, podcasts, a vast photo archive, 3-D image files and pages for missions where you can view raw data as they stream in.
• How stuff works   Expertly written articles explaining everything under the sun, from how a Taser is put together to the history of the jeep.  Hundreds of videos.
• Discovery Channel   Dozens of excellent microsites.  Links to the websites of sister channels (Military, Science and more).
• Smithsonian Institute   The Washington museum is the world's largest museum complex and the website is a rich source of eye-opening good reads.
• BBC - Nature  Possibly the best BBC microsite, offers a collection of video clips.
• TED  An organisation that invites inspirational people to an annual conference to share their thoughts on the world, technology, design and more.
• ARKive   Images of life on earth.  Endangered species are a specialty.
• History Today   Once registered (for free), you can browse through hundreds of articles culled from the popular magazine. 
• PBS   Publicly funded American channel with emphasis on science, the arts and history.  Watch entire shows or browse richly detailed sites.