OpenSkill is a fully open-source, peer-reviewed multiplayer ranking and rating system designed for building matchmaking systems. It offers functionality similar to Microsoft’s proprietary and patented TrueSkill, including support for features like partial play. Unlike TrueSkill, OpenSkill is completely free of patents and trademarks. It is fully typed, compatible with both PyPy and CPython, and maintains 100% test coverage.

A commonly requested feature that almost no n-player n-team rating systems have is the consideration of margin of victory and margin of loss. It's also known as "score margins". What are score margins? Almost every online rating system incorporates ranking information by using the ranks of player or by converting in-game scores into ranks. It doesn't matter if the opponent player wins by 10 or by 2 points. It's treated the same by most rating systems. This is what OpenSkill has recently solved. Simple systems like Elo and Glicko-2 can be modified to consider this, but it can't handle large scale battle arena matches accurately whilst being generalized to multiplayer multiteam settings.

Games currently using OpenSkill include: Hunt Showdown, MultiVersus and Beyond All Reason