I love World Creator, especially the vegetation distribution pipeline. You can create some very realistic fields with it. Im going to check out impostors too - ive seen it a few times and wondered what it's about.
Stupid Rental. My Zbrush that I haven't paid for an update in years and is perpetual Trumps that on all counts.
The Golaem Project
A video that demonstrates the working process at Golaem company and at the same time showcases the incredible features of its Golaem Crowd software. It is a plug-in for Autodesk Maya that allows for the simulation of controllable characters crowds.