Open Source Community Platform
A community platform lives or dies on whether members feel it's theirs, and nothing breaks that faster than a host that owns the member list, sets the rules, and can deplatform the whole group overnight. The open source engines here give you the full social stack - profiles, posts, groups, and reputation - running on infrastructure you control, so the network effect you spend years building belongs to the community instead of to a vendor's roadmap.
Discourse
Open-source community discussion platform with self-hosting, built-in chat, and themes
Rocket.Chat
Open source team communications platform with messaging, voice, video, federation, and apps
Forem
Open source software for building online communities, the platform behind DEV Community
Flarum
Simple, extensible forum software for building online communities
NodeBB
Node.js forum software with real-time discussions, web socket notifications, and Redis, MongoDB, or PostgreSQL storage
Lemmy
Decentralized link aggregator and forum for federated communities, with self-hosted servers
diaspora*
Privacy-aware, distributed social network with self-hosted pods and open servers
HumHub
Open source intranet and community software for communication, collaboration, and shared content
Stoat
Open source chat for friends and communities, powered by a Rust backend