The first time I did this was in the Vermillion campaign, where the characters were exploring the Minos Cluster setting. There, they found the planet of Eliad, populated by various exiled noble houses and guarded by an Imperial ship manned mostly by mercenary soldiers. It got me thinking. Noble houses. Mercenaries? I had always had a fondness for the Battletech game setting. Hmmm. Why not?
And so the noble houses on Eliad became those from Battletech: The noble Davions, the proud Kuritas, the stern Steiners, the reluctant Mariks and the treacherous Liaos. And those mercenaries in orbit? They were commanded by Jamie Wolf and Natasha Kerensky. Only one of my players (Todd) recognized the names at the time, but even then, it wasn't a detriment to play. In fact, it was a good hook for me to give the various nobles personalities that were 'ready made' and a lot deeper than I could have otherwise come up with on the fly.
I'm not certain how many other times I incorporated other settings into the campaign, but I know that I was always stealing from movies and books, including the often-maligned Marvel comics Star Wars series. I remember one time I had the players go to a planet that was essentially 'Transylvania' and battle a frankenstein-like sith-created beast. And again, it didn't seem out of place to have an 'old-world' type gothic setting. After all, it was just another planet, and that's just the way they lived. I tried to capture a lot of the 'Aliens' feel with the Otherspace adventures (but of course, they already had a lot of that).
Incorporating other game modules into Star Wars was quite easy, too. Star Frontiers modules translate with just a few tweaks. Even some D&D stuff can work, when modified: The 'Slavers' series is a good example, though again, it takes some finagling.
Ironically enough, it was only after everyone went their separate ways—and in fact only in the past few years—that I've really been brainstorming this stuff. You think of just about any science-fiction or action adventure movie and you can see ways it can be translated into Star Wars:
The Mummy? Hell yeah! A desert planet. A sail barge battle. A sith mummy.
Under Siege? (Bad movie, interesting concept) Terrorists take over a Star Destroyer of Mon Cal cruiser?
Planet of the Apes? Throw out the time travel aspect and its a perfectly good 'escape' romp.
The Black Hole? A Crazy scientist traps your group on a ship run by cyborg slaves?
King Kong! Why not? A remote world with a deadly ecosystem and lots of giant beasts! (Okay, maybe skip over the sad parts so SOME people don't cry too much...)
The list goes on and on. And though each of these ideas are 'derivative', WHO CARES, as long as they're fun!
In a different vein, I've often amused myself by trying to figure out how characters from other settings could work into the Star Wars setting. Imagine encountering the crew of the Farscape? Or the Serenity? Or even the Enterprise? As long as you ground those characters in the Star Wars universe, why not? I can see Kirk as a 'take charge' Rebel fleet captain. And then there are all the superheroes! Man, wouldn't it be fun to have the X-Men, only they're variant aliens and force-users instead of 'mutants'. The possibilities are intriguing...and fun to think about, even if I never do use them.
Yeah, just imagine: Wolverine: "May the force be with you, bub."