[Note added 2009-07-04: I now think that these were psyllids rather than aphids. See below for details.]
When I first saw these white fluffy masses I thought they must be some form of fungus, but then I noticed that one of them had legs and was walking:
These are woolly aphids, of the family Eriosomatidae in the order Hemiptera. They secrete a filamentous waxy white covering as a defence against predators. They also secrete droplets of sundew, several of which can been seen in these pictures. The tree is a common alder (Alnus glutinosa).
[Note added 2009-07-04: this post by Phil Gates, a botanist at Durham University, suggests to me that these are the nymphs of psyllids, in particular Psylla alni, the alder psyllid (family Psyllidae, order Hemiptera). Psyllids are closely related to aphids and they also secrete waxy white filaments.]
Photos taken in Reading University grounds, Reading, UK, on 2009-05-10.