Liz England, AAA game- and level-designer writes about theory and getting into the industry.
Dave Johnston, legendary level-designer who designed de_dust. Writes about multiplayer FPS level design.
Raph Koster, game-designer and writer of A Theory of Fun for Game Design blogs about creativity and game design.
Jon Jones, game-artist who has some great universal insights into making a killer portfolio.
Theory, articles and portfolios. A must for every level-designer / environment-artist
Looking for inspiration? Work? Feedback? Help? Go here.
Theory-heavy site about anything level-design.
A friendly Source SDK mapping community. The tutorials found here taught me Hammer.
All-encompassing article describing how to make a multiplayer level from scratch to the end for a realistic setting.
Level-design tips from the 'Sages' of level design. (Including: Levelord, Romero, Bleszinski)
Always insightful videos about a wide array of subjects important in the games-industry.
Daz walks through custom Quake and Source maps and expertly analyses them.
Like LevelHead, explains examplary examples of good game design.
James Partridge shares insights from his favourite Half-Life 2 map.