CrystalSpace is a portable modular 3D SDK, including many components for building various applications and games - these are still quite useful for projects that do not involve 3D. CS is still very much in development, so anyone who feels inclined to do so can contribute to the source. There are a number of incomplete components still needing to be finished, and a wide array of possibilities for new contributions; for more information about what is missing and a list of currently known bugs, please visit the Developer's Project page at SourceForge:
Crystal Space includes a foliage generator. This generator can randomly (based on user supplied input like density map and material based densities) place objects like stones, trees, grass, and so on. Especially in combination with our outside terrain engine this will make your worlds feel a lot less empty.