Here are a few sci-fi objects I have made for my Star Wars maps. I needed some equipment for the engineering section of a starship, so here are my reactors, generators, and conduits to connect them. They are pretty basic, but I usually "dirty" them up (with rust or chemical stains) when I place them on a map. [If you would like an example of how I have used these objects on my maps, see my Starship Sections thread. The Engineering section and Engineering overlays use these objects.]

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Also, we use some of my maps for online play. I have catwalks on several maps and needed a way to distinguish between who is up on the catwalk and who is down on the floor - hence a Level 1 marker to indicate that the folks on the catwalks are on Level 1 (instead of the Ground level). [I play Advanced Squad Leader and borrowed the Level idea from that game.]
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I don't know how useful that will be to those of you building maps, but it comes in handy when playing online with the maps.

I usually grab objects from The Holocron, RPG MapShare, WotC Forums, or Dundjinni Forums. If it isn't there, or I don't like what is there, I will try to draw something up for myself. (I do not see any use in re-inventing the wheel.)

As always comments, criticisms, suggestions, or bad jokes are welcome. Ok, maybe not the bad jokes.