Generally most software could produce a world like that depending on your skills, you're not going to do it willy nilly.
However, being one to complete that game, I know the game consists of a lot of foliage and terrian, as well as buildings. So my solution is.
Look at the tree, plant and terrian software on the TGC website, they might be quite useful, although they maybe a little realistic, you can pick up deled lite for free, lightmapping is a little low quality being the free version, but you can google 'lightmap maker' to lightmap in if you wish to use lightmapping. and you may want something like GIMP if you don't already own Photoshop or Paintshop(for texturing), The GIMP is free. Or you can get some of the freeby standard 3D modellers and do the trees, plants and terrian from scratch, then I'd go for Wings, anim8or or Blender for modelling the trees and plants and then terragen for terrian, which can export heightmaps for advance terrian if you're a DBP owner, if not I would go for .x terrian which you can still do in Terragen, as for the terrian software on this site, I don't know if they export as .x or just only as advanced terrian.
