Came across this site a few weeks ago and thought I would share it here. Basically, you can enter in sets that you own and this site will spit out other sets (and some MOC's!) that you can build with your existing collection. It prioritizes best matches, and will also show you how many/which pieces you are missing to create a given set. It also spits out interesting data on given sets and themes like most common parts, part type distribution, color distribution, etc.
DISCLAIMER: It's still being built, so the part/set reference catalogue is not yet complete. But most of the Star Wars line is accounted for, as well as plenty of Castle, City, and others. A recent look shows that they've added some more recent things like Toy Story, Architecture, Modular Buildings, and Pirates.