One of the big differences between running a fantasy campaign and a science fiction campaign is that when playing SF, I find myself constantly looking for starship deckplans.
With a fantasy campaign, many of the maps revolved around buildings, dungeons or overland adventures, and those sorts of maps were easy to knock out over lunch. Failing that, I had plenty of maps from 20+ years of Dungeons & Dragons that I could fall back on.
With my Star Wars campaign though, the adventures are split between world-based exploration and starship- or space station-based combat. Starship based adventures represent perhaps 1/8 to 1/4 of the encounters I run, but when I do run them I often find myself scrambling for deckplans.
The big reason there is that it takes more effort to come up with a rational-seeming starship. A dungeon can be as simple as a series of rooms, but with a starship players always want to know where to find the bridge, engineering, jeffries tubes, etc.
That's where ready-made deckplans help. The Star Wars rule books (regardless of edition) help with this, as they often feature deckplans for smaller ships, but the place where I consistently come up short is with frigate and larger vessels. I don't expect anyone to map out the Death Star (though I'm sure somewhere, somehow, someone is working on it) but a few detailed plans of light and heavy cruisers would be helpful.
I'm still searching for those, but over the last few years I have accumulated a goodly number of starship deckplan links. Here are some of the best, which I'll be featuring in an upcoming Knights of the Dinner Table column. If you know of a deckplans site that isn't listed here, please add it in the comments or email me at firstname.lastname@example.org and I'll add it to this list.
The Traveller Deckplan Page
Brook's Traveller Lowport
Peter's Traveller Page
Serenity RPG resources and deckplans
Colonial Chrome: Deckplans for Star Wars freighters, speeders, starfighters and capital ships.
Ki-Ryn: Future Armada: Extensive collection of starship maps, ranging from tramp freighters to space stations to battleships available for purchase.
Star Wars Deckplans Alliance: Lots of freighter- and starfighter-class maps.
Star Wars Maps: A collection of battlemaps and starship deckplans. Useful for minis games or RPGs.
The Star Trek LCARS Blueprints Database: A variety of Star Ship deckplans and blueprints.
Cosmographer 3: Traveller Starship Deckplans