What I think is sulphur tuft fungus, Hypholoma fasciculare, growing on a tree trunk.
Within a week of me taking these photos this tree was cut down. It stood beside one of the main paths through the Wilderness and I had noticed a year ago that one side was so badly decayed that it was probably going to come down sometime soon. Whenever I walked past it I would always speed up a little to minimize the chance of it falling on me. But now it's down, the log and branches will be good sites for finding wood-loving flies.
Photos taken in the Wilderness, Whitekights Park, Reading University grounds, Reading, UK, on 2011-10-05.