The basic gist of rebrickable is that you select a LEGO set, uploaded digital (LDD and LDraw) design (see eg. the rover we featured a while back) and it tells you if you can build it from your own parts. Which is pretty cool. Basically, it’s a library of sets and MOCs that not only shows you how to build things, but helps you find the parts you need to build them.
And it does all this from a slick interface. As a site, it’s useful for people who make their own designs (in LDD or LDraw), and for collectors. You can also compare two sets to see what parts they share. Or generate a part list from a CAD file and see how much it will cost.
What can you build? Rebrickable is there to help you answer that question.