These website are taken, mostly, from the Summer 09 Teachers Magazine published by the DCSF. A mixture of chemistry and physics.
Practical Physics - from the nice people at Nuffield and the IOP. Practicals for all occasions (with risk assessments too!)
The Naked Scientists - Podcast and linked website. Interesting answers to science questions, and articles about current science research.
Lancaster University Particle Physics Package - Aimed at A level Physics students.
Chem4Kids - American website which introduces children to the essentials of chemistry. Also available for other sciences.
Future Morph - Site to give students an idea about the range of science careers available.
Planet Science - extensive site developed from the Science year a few years a go. Lots of fun things to explore.
Web Elements - An interactive periodic table.