We switched to a green electricity supplier today. It's pretty hard to choose between them, because they all have such different approches. In the end, we went with Ecotricity because they were top of the listing at WhichGreen.org.
The listing compares the suppliers by the amount of new renewable energy that they're building, adjusted for the number of customers they have. Some resellers don't do any building, so they don't do very well in this rating. Ecotricity, on the other hand, topped this league for the third year running - they were 10 times better than the number 2!