Not strictly a bonsai in the classical sense, but nevertheless a tree in a pot. This little plant came to me from the Chairman of our bonsai club, who could not remember the actual species. A quick question on the Wee Trees Bonsai Forum and Paul from the Swindon & District Bonsai Society came up with the answer.
It's an alpine willow, Salix yesoalpina. The natural growth habit of this tree is to grow ground hugging, naturally cascading over rocky terrain. You see it here in spring, with the catkins just emerging. In the summer it is quite uninteresting, and the leaves are quite large.