Opening: Queen's Gambit

Introduction to Queen's Gambit:

Wikipedia about Queen's Gambit

Queen's Gambit Accepted
Wikipedia
The Queen's Gambit Accepted/Isolated Queen's Pawn (Exeter Chess Club)
    
    

       
       
Queen's Gambit Declined
Queen's Gambit Declined (Chess Corner)
Lectures by Steve Stoyko
Topalov - Anand, 2010 (Game 12) 
Vyzmanavin - Ruban, 1989 
Tarrasch (D40)
        
Queen's Gambit Semi-Slav
Wikipedia
David Vigorito: Play the Semi-Slav 
Adventures in the Semi-Slav 
Koneru - Danielian, 2013 

Queen's Gambit Slav
Wikipedia
Alekhine Variation 4...dxc4 5.e3