This has, by accident become the key chat/webmeeting software I use for my online games and organizing my in person games. We also use it to store the campaign journals for the games and randomly collect and add images, links and simple lame chatter.
Dragon/Dungeon Magazines (old paper)
Having subscribed to both of these in the last century and rediscovered the boxes of them late in the last decade, I find them to be one of the best resources for adventures (or adventure ideas, at least) as well as campaign and world ideas. Both are also available online for when I need to share information, maps, images with my online players.
Additional Websites
Great ideas, amazing character generator and the online store of 'official' DnD material
An amazing resource. Try to be careful not to spend too much money.
Matt's beginner DM video series is gold.
Johnn has tons of great tips and ideas and material. His Five Room Dungeons are perfect for a single night game or when the players head off on an unexpected tangent.
Justin Alexander's various 'remixes' of existing adventures and depth and sometimes a more cohesive story flow to some of the more well-known adventures.
Do not go here unless you have literally hours to spend creating maps. (I am not kidding)
This is an excellent collection of map extras, ideas, random generators. It also is linked to from Azgaar's above when clicking on one of the cities.