The most fascinating part of the war for me were the volunteers from across the world who joined up on both the left and right sides of divide.
Some good introductory links:
http://www.international-brigades.org.uk/
(the history section is the best bit)
Irish Volunteers
http://www.geocities.com/irelandscw/
Franco's Volunteers:
http://www.ess.uwe.ac.uk/GENOCIDE/reviewsw122.htm