I love
1632, the first in the Ring of Fire series by Eric Flint. The first and second books are available from the Baen Free Library. Here is a link to the first:
http://www.webscription.net/p-379-1632.aspx
Besides the regular books, there is also a series of anthologies with stories based on the characters from the series. Baen has a link to the entire series here:
http://www.webscription.net/c-10-rin...ric-flint.aspx
Also, Fictionwise has a huge anthology,
The Best Alternate History Stories of the 20th Century, edited by Harry Turtledove.
http://www.fictionwise.com/ebooks/b5...rtledove/?si=0
Fictionwise has a whole category for Alternate History stories, both multiformat and DRM titles. The cool thing is that you can find a lot of the classic short stories available as individual titles, without DRM:
http://www.fictionwise.com/ebooks/c7...nate-History/?