Cardforge (Forge, on their site) is a program that lets you build decks & play against AI (or humans) in many Magic the Gathering formats.
Their main forum can be found here: https://www.slightlymagic.net/forum/viewforum.php?f=26
Their Git repo is here: https://git.cardforge.org/explore/projects
Latest Desktop Version as of Apr 24 2021:
https://releases.cardforge.org/forge/forge-gui-desktop/1.6.40/forge-gui-... (60.1 MB)
Snippet from their FAQ to validate Free and Open Source:
The original programmer can be found at http://mtgrares.blogspot.com. A while back he open sourced the project and let the other developers improve it to their heart's content. He's taken a hands off approach for some time now, but still talks about it on his blog linked above. At this time, there are many developers who help in different stages of the game. Some of us work on the infrastructure of the program, others work on creating new abilities and cards that use them, but there is a lot of behind the scenes action going on. For each release, it is common for the release developer to give a shout out for those that helped specifically for that version. Feel free to give kudos there.