[FraKtus]

The best for me is artmatic: http://www.artmatic.com There are presets of trees that makes it easy to generate fine visuals and the primitive themselves are finetuned to generate impressive results. This is catch with that type of generators, giving the right sets of primitives, if you are too detailed it can take lomg to implemente something. Eric Wenger, the author of artmatic, is both an artist and a coder, there are not many creative peoples like him.