I use minis in my games, and sometimes I like to have scenes that will stick in the players' minds. For those I build some cardstock terrain. PDFs have the advantage of being inexpensive, easily customized, and if someone destroys your terrain then you can generally replace it. And the more terrain you have the more you can mix and match.
There are some good companies out there - with World Works Games
being my favorite, they make a good sized line of modern urban terrain, though the Invader flying saucer from Ebbles
also sees a good deal of use.
Ebbles makes some good SF terrain and vehicles while World Works has First Light, a set for building space ships.
Ebbles' vehicles are very nice, though I have only built a very few.Fat Dragon Games
makes some, ummm, strangely familiar fighter craft for sci fi gaming as well. I feel that they work well with the World Works First Light set, and if I am ever running an old style Battlestar Galactica game I will know what to use.
I would have included some images, but testing that showed how very large some of those pictures are.
Now if I could find some PDFs for Steampunk I would be all set.
The Auld Grump
*EDIT* I have been adding vehicles as well as terrain, so I changed the title of the thread.