I have forgotten to say this ;)
for PC he has made this one!
http://algorithmicbotany.org/virtual_laboratory/



Vue is very more easy than Terragen! And it's also a modeler for any thing! ;)
And many more complete and versatile and funny ;)

you have a free version of Terragen 2.5 for test it! ;) http://www.planetside.co.uk/forums/index.php/topic,16001.0.html