The number of Scandinavian folk music, dance, and cultural resources is large and growing.  It is only possible to touch on some of the highlights here -- my apologies for any glaring omissions.

This list is starting small but will expand.  Please contact me if you notice any errors or would like to suggest additional links.
Many of these websites are broad in their coverage.  Some are focused on particular instruments, others are regional in nature, many performing groups both in the US and abroad have sites, and of course there are other tune archives.
            
CD Roots 
Seattle Skandia Folk Dance Society 
American Nyckelharpa Association 
Austin Scandinavian Dancing
Lennart Sohlman's Swedish Fiddle Site 
Henrik Norbeck 
Los Angeles Scandia 
Hardanger Fiddle Association of America 
John Chambers' ABC Music Collection 
Leydon Grafix 
Scandia Camp Mendocino 
Nordic Fiddles & Feet 
Boston Speledans 
Scandia DC 
Scandinavian Fiddling Resources 
Northern California Spelmanslag 
Digelius Music 
Nordic Roots Radio 
Scandia New York 
Some Dalarna Spelmanslags & Links 
                  
NorthSide 
 

   
     
Nyckelharpans Forum 
Norsk, Ltd. 
Bjorn Bjorn 
Folketshus 
Swedish Folkdans Videos (via Dancilla) 
Northfield Minnesota Nordic Jam 
Dalarnas Spelmansforbund 
Alvdalens Spelmanslag 
Sveriges Spelmans Riksforbund (many links) 
Jochen Schepers' Swedish Fiddle Site 
Scandinavian Dancers of Vancouver 
Scandinavian Folk Dancing in Sacramento 
Chicago Spelmanslag 
Swedish Bagpipes 
Stefan Linden's Tunes 
Skanes Spelmansforbund 
Philadelphia Spelmanslag 
Nordic Heritage Resources (Kansas) 
American Swedish Institute