Just out of curiosity, I want to know opensource tools or projects that can add some interesting features to indie games, preferably those that could only be found on big-budget games.

EDIT: As suggested by The Communist Duck and Joe Wreschnig, I'm putting the examples as answers.

EDIT 2: Please do not post tools like PyGame, Inkscape, Gimp, Audacity, Slick2D, Phys2D, Blender (except for interesting plugins) and the like. I know they are great tools/libraries and some would argue essential to develop good games, but I'm looking for more rare projects. Could be something really specific or niche, like generating realistic trees and plants, or realistic AI for animals.