Brand new site Rebrickable has done the formidable, admirable, and unthinkable task of cataloguing loads of Lego sets and creating a database that can tell you what else you can build with them. Got a Rebel Trooper Battle Pack, the Mobile Police Unit, and the Quidditch Match set? Enter them into Rebrickable and see what new items you can build.
Rebrickable is a brilliant resource for Lego enthusiasts. Their main contribution is their giant database; it will tell you which sets you can build from the ones you own as well as how many pieces you might be short to build other sets. Then, the site will link you to several places where you can download the instructions to build these new creations.
In addition, though, Rebrickable also has a host of instructions for user-produced designs, called MOCs (an acronym for "My Own Creation") as well as a large collection of Lego stats and facts.
Check it out at Rebrickable.