There are several sites where you can download files to print for free.
*Update: Here is a site that has done a lot of the legwork and put together an active list of 33 of the best places to download models for free!
Another similar list is here at Ultimaker:
The one that is connected with MakerBot is explained in detail below, but there are other sites for you to look at including:
Using the resources for files presented within the pages can you find files for the following? Indicate where the files can be found with an appropriate URL
1. A game piece
2. A Star Wars character
3. A Sesame Street character
4. Something to be used with a pet
5. Something to be used on a desk