This is a plug in I've been lucky enough to have been a beta tester on for the last week or two.
What is it? To quote the T-Gen website:
T-Gen is the first tree and plant generator plugin fully integrated into XSI architecture. Over 100 appearance and distribution parameters, full utilization of XSI curves and node-based hierarchy ensures great flexibility and huge possibilities while creating your own trees and plants. You can use almost all XSI tools to additionally edit materials, geometry, hierarchy of all T-Gen objects. With powerful optimization tools you can create low-poly trees and plants for your games. Almost all parameters of T-Gen are animatable, so you can create animations of growing trees and plants.