The Diplomacy Archives collection of strategy articles.
On improving your play and “getting out of the box” (one of my favorite articles)
Chris Martin’s (1998 World Champion) YouTube Diplomacy Academy
DiplomacyCast country by country series (with special guests):
Austria England France Germany Italy Russia Turkey
Windy City Weasels Strategy Page
Diplomacy World‘s (original Diplomacy magazine) strategy archive.
The Diplomatic Pouch (online Diplomacy magazine)
Diplomacy Games podcast
Toby Harris’ (2015 World Champion) “Expert” Guides:
Austria England France Germany Italy I Italy II Russia Turkey
DiploStrats‘ YouTube channel (commentary on top boards and Diplomacy puzzles)