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.
TransitionKit: Open source scene-to-scene for Unity
TransitionKit allows provides modular, extensible transitions in-scene and between scenes to Unity SDK. This software contains several transitions for use in your games. The biggest plus of TransitionKit is that it allows to easily make your own transitions to give your game a unique look.