For the party of Fire and Dice, in case the main post gets buried for whatever reason.

For other groups (and personal reference):

Campaign Management Systems

For GMs to keep track of campaign events, encounters, lore, etc.

Character Creation/Management

For players to easily create and access characters online.

Maps - Creators and Software

For GMs wanting to use maps in their games.

Minis - 2D Paper Minis and 3D Printable Minis

For GMs playing in person and players wanting to make a custom mini of their character.

Music/Playlists

Collections of musicians and playlists to add the right mood to a scene.

Random Worldbuilding Generators

For general inspiration, mainly for GMs but might also spark some imagination in players as well.

Virtual Tabletops

For figuring out a good VTT for the group. Foundry and Fantasy Grounds are our current favourites.