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
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.