Aviones de la Guerra Civil Española: A superb site from Manuel Rodriguez Valverde with individual pages for many aircraft and with tons of graphics - The most comprehensive site on the Web. Mostly in Spanish.
L'aviació a la Guerra Civil Espanyola: Another great site, particularly for FARE aircraft (most of the links on the Nationalist page don't seem to work). Also contains a nice page of group/squadron badges, and lots of information on pilots, aerodromes etc. Mostly in Spanish.(This site seems to be no more, March 2004)
The FARE's fighters markings: A very promising looking spin-off from the above site. Although it has limited numbers of aircraft at the moment, and several non-functional links, there are some nice colour profiles of FARE fighters, with attack planes, bombers and "miscellaneous" to come. In English (This site seems to be no more, March 2004)
Cazas de la Guerra Civil Española: A site from Henri San Benito, covering fighters used in the Spanish Civil War, together with other interesting information. In Spanish.
Registro Español de Aeronaves: A site dealing with registration codes of Spanish civil aircraft, particularly interesting for its coverage of many of the civil aircraft that became involved in the civil war, with many pre-war and war-time photographs.
Potez 54 A site by Juan Antonio Soler, dealing not only with the Po54 in the Spanish Civil War, but with several other French aircraft types used by the Republicans. In Spanish and English (with a little French).
Breda Ba-65 A page about the history of this aircraft, particularly in WWII.
Spanish Civil War A site covering the Spanish Civil War as part of the UK schools National Curriculum.
General aviation links
The Military Aircraft Encyclopaedia (Russian) An amazing site with details of a good proportion of all the military aircraft that have ever flown - It's a pity that all the text is in Russian, but the pictures tell a thousand words...
The Military Aircraft Database A huge collection of information about military aircraft, 'though with few pictures. This particular database keeps moving site, so I hope this link will continue to work.