There used to be a tree kit freebie with several different trunk and tree meshes that you could mix and match.  I think one of the tree shapes was sphereical.  Unfortunately I can't find the website, but I think it was by the same person who had the extra primitives (star shapes, rounded cylinders etc.) and possibly also the torus-shell tutorial and the pyramid-diamond tutorial, if that helps anyone else find it.

Edit: Never mind about the tutorials, they're both peter sharpe's.  Thought I'd seen them both somewhere else first, though, especially the diamond one, with a lot more math.