links

PHET - a fantastic collection of interactive simulations for all ages.
Falstad - more advanced simulations, perfect for A level.
myPhysicsLab.com - more very detailed simulations.
Algodoo - extremely fun physics simulator, you build things and watch what happens. Algodoo can do mechanics (throwing things, friction, springs) and optics too.
circuit-diagram.org - draw neat circuit diagrams online.
powers of 10 - See what the universe looks like at different levels of magnification.
Daniel Russell - Acoustics simulations - A very detailed and complete collection of wave simulations explaining a very wide range of wave behaviour.