One of the online sites that I've used with great success for a long time is BBC Bitesize. This is an English site that covers a range of topics and subjects from primary to high school. I particularly like the way it has Revise --> Activities --> Test.
The activities are quite good fun; my particular favourite is the water cannon that teachers kids angles.
Here's the link:http://www.bbc.co.uk/schools/ks3bitesize/