Steam
now offers Allegiance as a free download, as Microsoft's multiplayer combat sim/RTS game from 2000 has been revived (thanks PCGamesN).

The game first died in 2002, but the source code was open-sourced, and FreeAllegiance has kept it going all these years.

This post notes the re-release, and says weekends will be the peak time to find other players while they build up their population.

A different post from last month outlines the project:

We are working feverishly on stomping the last of the bugs, and getting a fresh coat of paint on all the ships to get ready for our relaunch!

After 17 years, we are finally able to bring this game back to a major
distribution channel, which is a very exciting time for our entire community.

On behalf of the Free Allegiance team and the Allegiance Zone team, welcome to
Allegiance.

Allegiance is best played with LOTS of people, but it will be a little rocky at
the start as we try to attract players. If you want to help out, check back on
weekends around 2 P.M. ET/8 P.M. GMT and we will try to have a game running. Or,
grab a few friends and start up your own game. Nothing attracts players like a
running game.

In the mean-time, try out our training missions, and also read up on the wiki:
FreeAllegiance Wiki

There is a lot to see and do in Allegiance, and we welcome new developers to
join us on the open source project:

GitHub - FreeAllegiance/Allegiance: Allegiance - RTS + FPS Space Combat game

Source