Caltrop Core is a d4 tabletop roleplaying game engine.
The PDF 11 pages, with a bold and easy to read layout, and the mechanics are simple to learn and use.
Essentially, the way CalCore works is you roll a pool of d4s and count the highest. 3 is a partial success and 2 a partial failure, so as long as you're rolling multiple dice you have odds of making progress.
The engine explicitly supports both stat-driven and token-driven gameplay (think DnD vs Belonging Outside Belonging,) and it comes with a robust sample game built with the engine, so just by reading it you can get a glimpse of the engine's capabilities and range.
The sample game is fully playable, as are the tons of other neat titles that have been written using Caltrop Core.
Overall, if you want to try some low-number-dice tabletop design, Caltrop Core is a neat little engine and absolutely worth a look.