Wikipedia pages with fastfacts on:

   
NASA's STEREO Mission mainpage. I worked on education resources for this mission our the 'Space Observatories at School' project. The 'learning center' page is here (education resources). With more education resources here.
   
Near realtime info on the Sun from the Solar & Heliospheric Observatory
   
NASA's Planetary Data System page for information on the planets (fairly basic)
   
NASA's Goddard Space Flight Centre page for information on the planets (much more detail)
   
   
Mainpage for NASA's Mars Exploration Rovers. Image resources are here and videos are here.
   
Info on Russian plans to go to Mars
   
Link page to lots of NASA Mars resources for schools
   
Interesting article on some of the issues surrounding going back to the Moon and on to Mars
   

Mars based puzzles, funstuff and cutout models of Pathfinder, Global Surveyor and Odyssey. The same page on another site is here.

   

Imagine Mars website, more Mars based educational resources. Aimed more at KS2-3 than KS3-4 but the 'Mars Info Central' page is good.

   
Nice general space-info education website
   
Information explaining the gold disc aboard the Voyager probes containing a message to aliens.
   
NASA's Keplar mission to find Earth size habitable planets. There is information on Johannes Kepler and other PowerPoint files in the education section.