terragen, if memory serves, is still freeware, and if not, its likely a lot cheaper than Bryce. Also, rumour has it that Corel doesn't plan on introducing new versions of bryce. Both are terrain generators. Poser poses 3D objects that have been "boned" or have skeletons. No real model creation with it. Just th messing with (albeit, many) parameters. Rhino 3D is a nurbs based 3D modeller, with a slick CAD like interface (keyboard used as or more often than the mouse), which you either love or hate. For more info, infinitely more than anyone here can provide, take an hour and look through the official websites for them (which can be found using google).