Goat willow

Salix caprea is also known as the pussy willow because of its furry catkins or pussies.
Willows are dioecious and this one is a female with grey pussies.
It also provides equine services as a scratching post.
This was growing on Chailey Common and is quite a mature specimen. 

Grey willow (S. cinerea) is very similar but a key distinguishing feature is that if you peel the bark of a two-year old twig and run your fingernail around the wood you can feel ridges, whereas goat willow is smooth. 
I remember this by thinking of a smooth old goat. Why old goats might be so described is utterly irrelevant.