ORB – Online Reference Book for Medieval Studies – very good reference site overall:
http://orb.rhodes.edu/The Labyrinth – Resources for Medieval Studies, sponsored by Georgetown University:
http://georgetown.edu/labyrinth/

Internet Medieval Sourcebook – selected sources and texts; scholarly:
http://www.fordham.edu/halsall/sbook1g.html

Rosslyn Chapel Trust – the official Rosslyn Chapel website, including news, beautiful images of some of the carvings, and replicas and books from the online shop:
http://www.rosslynchapel.org.uk

WWWVirtual Library, History Index for Medieval Europe – vast info. here:
http://www.msu.edu/~georgem1/history/medieval.htm

Medieval History Time Line –
http://ukans.edu/kansas/medieval/timeline.html

Medieval Maps and Images –
http://www.fordham.edu/halsall/sbookmap.html

Medieval Manuscript Manual – structure and manufacture of medieval books:
http://www.ceu.hu/medstud/manual/MMM/index.html

Knights and Armour – many related sites listed here about knights, chivalry, armour:
http://historymedren.about.com/cs/knightsarmor.index.htm