Top 4 C++ cross-platform-game Projects

openspades

6 1,098 2.9 C++

Compatible client of Ace of Spades 0.75

Project mention: Open source Diablo 1 engine – DevilutionX 1.5.0 released | news.ycombinator.com | 2023-06-17

Ace of Spades is one. I usually play with the OpenSpades client[1], but there is also Betterspades[2], and probably many other clients I don't know of yet. There are usually about 10-50 players online on the public servers listed on BuildAndShoot[3], variable depending on the time of day and mostly from Latin America it seems.
One can host the game with piqueserver[4]. I'm not sure if one can still host with the original Ace of Spades server, but the game was 'shut down' in 2019 so maybe not.
[1]: https://openspades.yvt.jp/
[2]: https://github.com/xtreme8000/BetterSpades
[3]: https://www.buildandshoot.com/servers/
[4]: https://www.piqueserver.org/
performous

6 469 8.4 C++

An open-source music and rhythm game.
WorkOS

workos.com
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The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.
bzflag

1 280 7.5 C++

3D multi-player tank battle game
Moondust-Project

1 243 9.4 C++

Moondust Project by Wohlstand

NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020). The latest post mention was on 2023-06-17.