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.
Creating Topological Art with XGen
This is a video demonstration of using XGen in creating simple cubes and adjusting their heights and color with via ptex map to create interesting works of art.