Here are some for the Year 12 Astrophysics students:

Powers of Ten
Sten Odenwald's Questions & Answers
More from Sten Odenwald
Stellar Evolution
Ned Wright's Cosmology
Space Science Education
The Bad Astronomy Page (try the links)
Urban Myths Debunked
BBC AS Guru (No Physics yet, but worth visiting)

Here's a selection of other science links we have found useful (special thanks to Year 9 for their research):

http://micro.magnet.fsu.edu/primary/java/scienceopticsu/powersof10 (not to be missed!)
http://www.bbc.co.uk
http://www.bbc.co.uk/education/gcsebitesize
http://www.bbc.co.uk/education/asguru
http://www.omnia.com
http://van.hep.uiuc.edu/van/qa/qaform.htm
http://www.eb.com (Encyclopaedia Britannica)
http://www.pinholecamera.com (light and photography)
http://www.looksmart.com
http://www.kapili.com
http://www.darvill.clara.net (radioactivity)
http://www.science.com
http://www.channel4.com/homework
http://www.exploratorium.edu (astronomy)
http://www.physics4kids.com
http://www.energy.ca.gov
http://www.cornwallis.kent.sch.uk (electricity)
http://www.chem4kids.com
http://www.schoolchem.com
http://www.thinkquest.org/library/index.html
http://www.physicsclassroom.com
http://www.gcse.com
http://www.tang.btinternet.co.uk (Chemistry)
http://www.iop.org/ (Institute of Physics)
http://www.madscientist.com
http://www.nasa.gov/
http://www.hawastsoc.org
http://www.newscientist.co.uk/
http://athena.wednet.comu
http://www.hawastsoc.org/solar/ (solar system)
http://volcano.und.nodak.edu/vw.html
http://www.accessexcellence.org/AE/AEC/CC/historical_background.html (radioactivity)
http://www.psigate.ac.uk/vts/physics/index.htm
http://www.energychest.net
http://earthkam.sdsc.edu (space shuttle images)
http://oposite.stsci.edu/pubinfo/subject.html (Hubble telescope images)
http://www.carolina.com/physics/
http://www.walter-fendt.de/ph14e (simulations)
http://www.fearofphysics.com
http://www.howstuffworks.com