http://www.bhg.com/gardening/plant-dictionary/tree/. – an alphabetical list of trees, small, medium and large. Most of the trees are US centric though.
http://www.british-trees.com/treeguide/view_all_species.htm comprehensive list of trees in Great Britain, compiled by the Woodland Trust.
http://en.wikipedia.org/wiki/List_of_trees – Trust who else but Wikipedia to give you a comprehensive listing of trees. There’s even an Asian section.