Centuries ago, before the galactic
currencies collapsed, the planet builders of
Magrathea built planets to any
specification, assuming the clients had enough money to pay for their services, that is. Planets made of
gold, planets just the right shade of
pink, planets with just the right length of day at the
equator, planets with short periods of rotation to give the occupants more
birthdays, all of these spring forth from the great
hyperspatial factory of the Magratheans.
For those of you who can't afford to have the corporate planetary building might of Magrathea creating their dream world, there is Bryce 4 by MetaCreations, the makers of Poser 4.
The fourth version of the famous planet creation program is the best yet. It allows you to specify sea, rocks, terrain, clouds, and other objects like buildings, along with the lighting and camera angle, to create a wonderful landscape, and will even let you create walkthroughs and animations. Some of the features look stunning, especially when you use alpha channels, transmissive and diffuse colours, and specularity and reflective colours. You can create smog effects, glass, realistic water, and other such graphical wonders.
Be warned, though, this is still pretty slow. Fully rendering a raytraced landscape in Bryce 4 can take hours, but doing an animated flypast or walkthrough can keep your computer busy for days, or even weeks...