Daz3d had released Bryce 5.5 free without any limitations back in 2006. They no longer offer the download on their site but a quick google search netted me a number of pages which still have it.
It is over 120 Mb and well worth the download if you are interested in a tool that can do as much and more than Terragen. You need to be willing to wrap your head around a non-intuitive interface, but once you have it is quite versatile for skyboxes and the like.
It also has fairly advanced animation tools which are well integrated into the package. Take some time to go through some tutorials and don't let the poor interface deter you from really getting into it.
