Zach Smith and Bre Pettis have created a web site called Thingiverse, which is designed to make it easy for people to share digital designs of real objects. I have put most of the things I have designed for printing on a RepRap on there. I even created one especially for it, a key for reading utility meters :-
You can get these free from the utility companies, but not a quickly as you can RepRap one for a few pennies worth of plastic, and they are easily lost. The files can be found here :- www.thingiverse.com/thing:88.
I also keep a gallery of all the things I have made with HydraRaptor here: sites.google.com/site/nophead/Home/hydraraptor/ThingsMade. They all now have links back to Thingiverse for the files.
A lot of the objects on Thingiverse are for a laser cutter, but I downloaded this twisted star box designed by Marius Kintel and printed it.
One trick I have learnt is that you can shrink objects a little by heating them with a hot air gun. When I first made this the top was too tight, so I shrank the bottom a little with the heat gun. It is now a perfect fit. You have to be careful not to get it too hot, or it will sag.