Due to internet being a dynamic place, data here may be outdated. Please report any dead links or new unlisted sites and servers in the comments below.
This is a short list for the purposes of quick reference. A slightly more wordy state of affairs description can be found in Current State of Affairs.
Web sites and game hubs
The Origin of the Freecivs. This is where it all started, although it is more of a technical site and less of a community hub. Contains forum with all technical announcement about native client ("Application"/"Program") versions. Obviously, this forum is also considered the central communication nexus for all sub-communities.
The inventors of the longturn "sub-genre". Established in 2003, requires "native client", or application that can be downloaded from several sites. Ruleset development and very inclusive environment. Organises games with several ruleset branches. Aiming to develop a stable web version. Welcomes developers at all levels.
Browser version of the game, concept started in 2017, this web site active since 2019. No downloads needed. Runs only one ruleset branch.
Tactics & Triumph
A spinoff from Freeciv Web, 99% identical, but developing its own ruleset branch in order to add variety to the game.
Web version of the native client (no download needed), still requires some fine-tuning and final touches, but the most prospective technology at the moment.
An attempt at web 3D version of the game. Doesn't offer multiplayer yet.
Web-based. Temporary/unofficial/private project; by the words of the owner, "a pet project and no one should depend on it being alive tomorrow".
Main communication hub for Longturn.net: ruleset development, ideas exchange, longturn game announcements, discussions and analysis, exchange of ideas, newbie support.
League of Civs
A purely community server, not attached to any game variants.
Community support for Freecivweb.org site.
Tactics & Triumph
Community support for T&T site.
Reddit Freeciv section
For passionate Redditers out there.