Reddit page with tons of links
https://www.reddit.com/r/gamedesign/wiki/index

Knowledge base with links to articles, videos, etc.
http://level-design.org/?page_id=2468

Research compilation (design thinkers, books, tutorials, etc.)
http://gamedevprofessor.com/research-compilation/

Reverse design of games (eg. Half-life, Super Mario World)
http://www.thegamedesignforum.com/

Blog organized as a course on Game Design
https://gamedesignconcepts.wordpress.com/

entire game design education that you need right in this post
https://www.reddit.com/r/gamedesign/comments/33h8ql/how_do_you_guys_design_games_whats_actually_going/cql9ffk/