Options for getting a professional-quality prototype of your board or card game printed:

https://boardgamedesigncourse.com/the-top-5-places-to-get-your-board-game-prototype-made/

Free customizable, printable templates for papercraft boxes and shapes of all sorts:

https://www.templatemaker.nl/en/

Resources for buying or making hex grids for prototypes:

https://www.reddit.com/r/tabletopgamedesign/comments/pulf6z/hexagon_map_prototyping/

Data visualization and board games: how shape, color, layout, and more help immerse players into the game (videos):

part 1: https://www.youtube.com/watch?v=K2n9jZMaK9o

part 2: https://www.youtube.com/watch?v=eh_wMZaF-b0

Programs for designing minis and figurines for tabletop games:

https://www.reddit.com/r/tabletopgamedesign/comments/pmfejz/what_programs_do_you_use_to_design_figurines/

Tons of health and medicine icons, free to use for your medical-themed prototype:

https://healthicons.org/

Quick discussion on the pros and cons of using virtual game simulators as a design tool (audio)

https://tenminutedesignchat.podbean.com/e/episode-24-digital-tabletops/

Designers on which printers are good for making prototypes—a Twitter board game design discussion:

https://twitter.com/JVDesignsGames/status/1408220087639756805

This browser-based photo editor has all the functionality of similar, more expensive software, but for free:

https://www.photopea.com/

The basics of how to make a board game prototype: bits, boards, and cards, plus how and when to make changes (video, 16:25-33:45)

How to make tiles for your board game prototype—without getting your hands sticky with glue (video)

Graphic design for board game designers: fonts, numbers, card layout, color meanings, graphic design programs, and more (video)

This tool helps you count and visualize all the possible permutations for tiles (or rings) based on whatever parameters you choose:

http://burnside-calc.com/

Lots of meeples of various shapes for Tabletop Simulator:

https://steamcommunity.com/sharedfiles/filedetails/?id=644617425

How to create a custom table in Tabletop Simulator:

https://www.breakmygame.com/post/tutorial-making-custom-tables-in-tabletop-simulator

Board game designers share tips and things they wish they had known about creating a project in Tabletop Simulator for the first time:

https://twitter.com/PartyHatGames/status/1380197305492500486

Options as a stand-in for punchboard for prototypes or print-and-plays:

https://twitter.com/dustinbschwartz/status/1376792170246639616

These Google icons are open source and free for everyone to use:

https://fonts.google.com/icons

What’s the best way to prototype custom dice? A Twitter board game designers discussion:

https://twitter.com/playnyctophobia/status/1365884829590519808

This tabletop-game-themed typeface is available for pay-what-you-want:

https://speakthesky.itch.io/typeface-dicier