The folks over at www.toolstoday.com have a list of some sources for free (and not so free) patterns and models to get your DIY CNC router/mill in production mode. Some patterns / models may be just for personal use. Please check the rights of use, especially if you intend on using for items to be sold.
The folks over at www.craftsmanspace.com also have a bunch of 2D patterns and 3D models. Of course, they sale their own machine designs and offer ressources, some free, some not.
Their patterns and models appear to be free to use for personal use. If you intend on using for commercial (for sale) items, contact them first.
Meanwhile, take a look, as I am doing right now, and see how you can test your CNC router/mill with more than just lettering or typical circle/square test patterns.
Have fun ...
Lots of "free" 3D models and 2D patterns 2015-12-22
Here is a list of libraries including free 2D patterns and 3D models to get you going ...