These are the sample files used by Comtech Services in their Introduction to DITA, by Jennifer Linton and Kylene Bruski.
Buy a copy of the book ($50 from Comtech), work through all their chapters, and do the exercises online here. You don't need to set up the DITA OT on your machine (a major hassle for some people).
These are the original sample files provided by IBM In 2003. They illustrate the use of a frameset and related links.
These too are the original sample files provided by IBM In 2003.
Now - join and get a free copy of JoAnn Hackos' Introduction to DITA or an <oXygen/> XML editor with WebDAV access to your online workspace folder.