Some links that I have found useful and interesting:

Archives, Libraries and Museums
Swedish Krigsarkivet
Swedish Riksarkivet
Swedish Army Museum
Swedish Kungliga Bibliotetek
Danish Rigsarkivet
Danish Tøjhus Museum
Norwegian Riksarkivet
Norwegian Forsvarsmuseet
Swedish Archive Information Service
Chakoten (Danish Military History Society)
Royal Danish Garrison Library
Swedish Armemuseum Trophy Collection

Battle and Order of Battle Information
The Nafziger Collection (Period Orders of Battle)
Battles of Great Northern War
Swedish Battles
Nick Dorrell's Great Northern War Pages

Books and Magazines which cover the Period
Acedia Press (Books and other useful information on Swedish Uniforms, Colors and Standards)
Gauntlet Publications (Source for Russian books and magazines on uniforms, etc.) 
Pike and Shot Society (Publishes the magazine "Arquebusier" which has many useful articles dealing with Cossacks, the Polish-Lithuanian Army and the Russian Army, as well as the Danish and Swedish Armies - highly recommended)
Årnfelts förlag (Gå På Rules and Army Lists)

Russian Galley Fleet 1714
Russian Navy History

Reenactment Groups:
Fredriksten Artillerie Compagnie
Nerike Wermlands Caroliner

Kong Fredrik IV Tambourafdeling av 1704
Polish Artillery
Preobrazhenskt Life Guards

Arkeliet ( Danish-Norwegian Artillery and associated equipment, tools and ammunition)
Christiansteen Artillerie Compagnie
Smaalenske Regiment
Karl's Compagnie
Blachiersche Compangie

Norwegian Fortresses

Fredriksten (Halden)
Kristiansten (Trondhjem)

Other Worthwhile Sites:

1713-1714: The Catalans' War
Wars of Louis Quatorze


Northern Wars