This thread is specifically for the sharing of resources. Both One-Page-RPG-specific resources and general-RPG-design resources are welcome!

Below are a few I think might be helpful, but please add any more you think would be useful to people in the comments!

Submissions to the One-Page RPG Jam 2020

Submissions to the One-Page RPG Jam 2021

TTRPG Resource Jam - All submissions to the TTRPG Resources Jam with a wealth of templates and advice.

Open License One Page RPG Systems - A list of one-page systems with open licenses allowing you to hack the rules for your own game. (Courtesy of Chaosmeister.)

Game Icons - Free icon graphics.

Safety Tools - A description of various social and content tools for TTRPGs. 

Any Dice - Dice probability calculator.

Itch.io Image Templates - A set of templates to make images the correct size for various places on itch.io.

/r/onepagerpgs - The one-page RPG subreddit

/r/RPGcreation - General subreddit about designing ttRPGs.

https://newschoolrevolution.com/public-domain-art - Nice list of places to get public domain art :) !

Edit: please be cautious and make sure the art you get from sites actually is public domain! Some sites do better than others.

I wrote a handy one-page (OK, two pages, front and back) guide on emotional safety techniques you can include in the rules of your games
https://nadg.itch.io/emotional-safety

Although  normally used to make D&D5E homebrew, this website by Natural Crit  allows you to make complete, professional looking tabletop homebrew modules in your browser. It is easily modified and highly customizable, and will be an extremely useful tool to anyone without access to professional software from Adobe and the like.

The Homebrewery

I have an SRD up that includes a single-sheet template for your games.

Since its release, we've seen near 1,300 downloads and nearly 60 games made with it.

Push SRD by Cezar Capacle (itch.io)

I have a list of TTRPG Resources here. :)

If you want to make your one-page game look more like a game on a page, you can check out my The Paper Project. Some of the images there were made for my own use in games during this jam!

Can I offer up my Canva templates? Just in case they might be of use!

https://harper-jay.itch.io/canva-templates

Hello game developers, writers and table top players, 

we have recently created a Unity asset which is a dice generator. It can be used for TableTop and RNG games. Great for playing TTRPG as single player when you don't have dice available.

It is open source and freely available, can roll any random number of dice and range, percentages and alphabets.

Zeus Unite - Dice Generator (Free) by Stusse Games (itch.io) 

• Dice Generator can be played for free in the web browser and is available as a download for Windows and Linux.
• Dice Generator is also a Unity asset that can be used to create TableTop and RNG games within the Unity engine. (Soon available for free in the Unity AssetStore).