Update October 1, 2018: for a good, fairly comprehensive and freely downloadable set of sample DITA documents, I highly recommend checking out the list on the OASIS DITA Adoption website, compiled by Stan Doherty. Depending on what you are looking for, these files are great for learning more about the practical aspects of using DITA.
If anyone knows of other DITA source code examples, please add them to the comments section below.
The open source DITA2Wiki Project comes with a small set of DITA source you can play with. (The project distrib enables publishing from DITA source to wiki – Confluence, out of the box).
http://sourceforge.net/projects/dita2wiki/
The Codex online help source files are publicly available: http://www.codex-systems.ca/help/CodexHelp.zip
@Adam and @Lisa: thank you for the additions — much appreciated!
VR Communications has posted the source code for our 350-topic DITAinformationcenter, Grocery Shopping sample, and Garage Sample (enhanced version of the Garage sample that comes with the Toolkit) at http://www.vrcommunications.com/downloads.
(The published versions of these DITA documents are posted on DITAinfo.info and XMLdocs.info.
Thanks for adding that one Anna!
The documentation link to the Apache Derby samples mentioned by the OP Ditawriter seems to have moved here: http://db.apache.org/derby/manuals/dita.html