There are so many online games out there and so many deserve a look, so it seems a bit fair to single out only one… but it has got me hooked:
Just click the little ball you want to start and it will explode into a circle of light – if it touches any other balls they too will explode. The aim is to get as many exploded as possible.
I am up to level 12 and will get past it soon!
Edit: Finished the game, final score 253!