In order to print an object from a 3D printer you need a 3D model file, generally an .STL file. Where can you find .STL files? Three places. You can make one yourself, using 3D modeling software like AutoCAD or Google Sketchup. You can scan an object in the real world and convert it into a 3D model (I hope to try this soon). Lastly, you can download an .STL file someone else has made.
Following are the majority of the designs I printed (or tried to print) during the course of reviewing the MakerBot Replicator. Most I obtained through Thingiverse, MakerBot's community-driven repository of 3D object design files, all of which are available to download for free. With 15,000-plus design available on Thingiverse, I barely scratched the surface of its content.
Of the files I pulled from Thingiverse, this is the most impressive item I printed successfully. The cube-shape and the ball come out in a single print thanks to the Replicator's dual-extruder printer head (or "dualstrustion," as MakerBot likes to call it). Design by etrohn at Thingiverse.com. This would make a great cat toy.