Below are a number of resources I’ve used for worldbuilding projects in the past. These are personal recommendations; I’ve received no free copies or payment for these resources, tools, books, and games listed here. This list will continue to grow and change over time.
-T Hergenrader

Online Resources
World Anvil Worldbuilding Tool
Google Docs

Online Mapping Tools
ArcGIS
Google Maps
Mapbox

Books on Worldbuilding
Building Imaginary Worlds: The Theory and History of Subcreation, by Mark J.P. Wolf
Revisiting Imaginary Worlds: A Subcreation Studies Anthology, edited by Mark J.P. Wolf
The Routledge Companion to Imaginary Worlds, edited by Mark J.P. Wolf
Third Person: Authoring and Exploring Vast Narratives, edited by Pat Harrigan and Noah Wardrip-Fruin

Worldbuilding Role-playing Games and Resources
City Builder: A Guide to Designing Communities, by Skirmisher Publishing
Dawn of Worlds, by Legends
Dungeon Master’s Guide (D&D Core Rulebook, Fifth Edition) by Wizards of the Coast
Dungeon World, by Sage Kobold Productions
Kingdom, by Lame Mage Games
Microscope, by Lame Mage Games
Non-Player Cards and Compendium, Metal Weave Games
Worldbuilding Theory and Setting Design, by Dancing Lights Press