A short story set in the Old Man’s War universe (in which 75-year-old citizens of Earth are recruited to join the defense forces of human colonies in space), published online at Tor.com.
Available from: http://www.tor.com/index.php?option=com_content&view=story&id=49
(Read online or use the “download” link on the left side of the page.)
An index of stories from Tor.com, all of which are free and downloadable, is here.
John Scalzi has a sampler of free creative work, including the full-length novel Agent to the Stars, available here. However, most of the works are not in downloadable e-reader format.