I came across these (http://www.digi-element.com/site/products.htm) in my browsing. Aurora looks the most interesting to me, has a nice way of adding skies and water effects. Verdant looks fascinating, to add photoreal plants, but the individual plants are very expensive (though infinitely reusable). Modelshop could also be fun, lots of free models to import on the internet. Not cheap, but they have free demo downloads. Aurora might be worth it, and has "movie" tutorials on the site.

Nice too:

redfieldplugins.com

Nice plugins. Thanks.

I'm looking for a way to figure how must look an object's shadows.
Imagine you have to place a car or a table over a new background and must to draw the shadows according to the new scene.

I'll see if "Model Shop" may help with this subject.

I bought Aurora (but you can play for free for 30 days). This took about 3 minutes using ducky.tif (you should watch all the tutorials on the site before installing, there's not much of a learning curve, but the first bump is a steep one).

This one took a little longer. I used a preset, but made the moon larger, changed it's phase, moved it to the left, then edited the clouds a bit.

Thanks Doug!
Nice samples. Will try Aurora.