General Metalwork and Smithing Links:
The Artist-Blacksmith Association of North America 
The British Artist-Blacksmith Association
Anvilfire.com - A great resource for blacksmithing on the web.

Medieval and Renaissance:
The Society for Creative Anachronism (Or the S.C.A. for short) Probably the largest and most extensive Medieveal /Renaissance reenactment organization in the country.
Renfaire.com - All things ren-faire.
Medieval Life and Times - Information about medieval life.
The Armour Archive - EXCELLENT resource for armour enthusiasts.
The Knighthood, Chivalry and Tournaments Resource Library

Vikings, Saxons & Norsemen:
Fjellborg Vikings - A Colorado based living history group portraying the Vikings, Saxons and Norsemen  (c.793-1088 A.D.)
The Vikings - A parent organization portraying the Vikings, based in the UK, but with groups worldwide.
The Vikings Vinland - The Canadian branch of the above orgnaization. 

Greeks, Romans, Celts and The Ancient World:
Legio XX - A premier website on Roman living history
RomanArmy.com - Roman Resources
The Roman Hideout - Great page! All things Roman/Ancient history.