The Why Files
How Stuff Works: which also has sections on
science and on computer stuff.
Then there are the BBC, ABC and channel4 science related websites:
ABC Science have a good look around as there are links for many things on this front page for ABCScience
Channel 4 Science including some good online learning stuff
Channel 4 Science for Kids with areas for specific age groups:
BBC Schools oriented Science UK-centric but only some of it.
BBC Science & Discovery has a newsletter and message board:
BBC History Newsletter with a scientific aspect
BBC Learning Science