Slack

Slack wasn't built for public communities — it's workplace chat software — but plenty of newsletters use it anyway for a subscriber community, mostly because it's a tool people already know how to use and it's free at a small scale.

The limits show up as a community grows: Slack's free plan hides message history after a while, and the interface isn't really designed for public discovery or drop-in browsing the way a dedicated community platform is. It tends to work well for smaller, more intimate groups and less well once a community gets large.