Found out about Rebrickable via a post on Kottke.org complaining about the price of Star Wars Lego which linked to a post on Reddit where a comment mentioned it:

Rebrickable will show you which LEGO sets you can build, by reusing the sets and loose parts you already own. You can choose from official LEGO sets or custom made MOCs (My Own Creations) by many different designers. All MOCs include building instructions, some of which are even better than the official LEGO ones!

Awesome.