I've got a few Op-Tech Rainsleeves, which were about a fiver a pair, and they are pretty good. They are roughly the correct shape of the camera and they do one for a flash on camera too. I've used the same one about half a dozen times now and it's still as good as new.
The Think Tank range may well be a lot better but they are also a lot more expensive, but maybe worth it if you're out regularly in bad weather. For me, usually when it rains I stay in and watch it from my window... I don't like getting wet.