That's Honey fungus if I'm not mistaken. Often grows on tree stumps, but where it grows on a living tree it damages the tree and may kill it. :(
@Matt - You are quite likely right. I'm by no means an expert mycologist, and am actually embarrassed to admit is one of the few field guides missing from my bookshelf. I'll need to look into this further!
I use Roger Phillips' "Mushrooms and other fungi of Great Britain and Europe", which is very good, and I see that he's also done a "Mushrooms and other fungi of North America". Looking at your photo again I'm now leaning towards Sulphur Tuft - I can't see a ring on any of the stems, which honey fungus has. This is why I don't eat most of the fungi I see - there are so many cream-grey or yellow-brown species and it's easy to misidentify them. In practice I only eat blewits and Chanterelles - both very distinctive.