Wildfire Games 0 A.D. (https://play0ad.com/) is a surprisingly rich multi-platform free open-source game that's been in alpha state for almost two decades now (see https://play0ad.com/about/the-story-of-0-a-d/) and I reckon it's about time it gets a post here at TPFC.
0 A.D. name and development status make it notoriously difficult to search for, but it has seen a couple of PortableApps.com releases over at https://portableapps.com/node/24356 (see also wildfiregames.com/forum/?/topic/13254-0-ad-portable/).
The latest official 0 A.D. release is "Alpha 23 Ken Wood", dated 2018-05-17: play0ad.com/new-release-0-a-d-alpha-23-ken-wood/.