The alt-history RTS World in Conflict was released in 2007 by Massive Entertainment, and was very well-received, with high review scores, several "strategy game of the year" awards, and impressive initial sales. Despite that, it never got a full sequel (an expansion, Soviet Assault, was released in 2009) and while Ubisoft kept the multiplayer servers running for years after it acquired Massive, in early 2016 it finally pulled the plug.
Earlier this month, Ubisoft made World in Conflict: Complete Edition free (initially until December 11, although it's since been extended to December 23), and now it's gone one step further by making Massgate, the multiplayer server software, open source. That means that anyone who wants to can take, use, and modify the software, without restriction or charge.
"The code itself is more or less the same as how the code looked like back when the game is released. Only minor tweaks have been made to make it build on a relative modern compiler and to remove the necessity to manage CD-keys," the GitHub intro message says.
"Not much of the code has survived into later releases done by Massive Entertainment and does not really reflect to code of the company today, apart from the code standard and the general look and feel of the code. As a piece of game development history, and for anyone interested in how online servers were written at the time, it can definitely be a point of interest."
To be clear, this is not the game but the backend that the enables multiplayer, so you'll need MySQL, CMake, some version of Visual Studio, and of course knowledge of what to do with all of that. For anyone who wants to take it on but could use a gentle nudge in the right direction to get started, the page also includes information on building and running Massgate, a web server, dedicated server, and even some suggestions for possible improvements.
It's all well over my head, but people involved in the community-run Massgate project aimed at restoring multiplayer functionality seem over the moon about it: One commenter on the subreddit called it a "wonderful Christmas present," while another described it as "absolutely glorious."
The open-source Massgate is available for download from GitHub.