Retopo tools ?

Although i use 3dcoat a lot, i never tried the automatic retopology in it, so i can't tell you if it's relevant or not; but maybe you would want to give it a try

http://3d-coat.com/retopology/

i don't know if it works well for environment stuff tho

3dcoat is really awesome but I think Decimation Master still gives better results when it comes to retopology!

Zremesher in Zbrush, great results!

3DSMax's Multires :>

<runs away>