Rites of War is fairly good game, made on Panzer General engine, it features Eldar vs Tyranids conflict. Good old times.
Crusader Kings II is free for some time now, to lure unwitting souls down the rabbit hole of DLCs, that are legion. Vanilla game is OK, but it is a considerable time sink.
Few freeware games:
MegaMek is electronic version of BattleTech tabletop game. I have a slight feeling I am not the only one on this forum that is familiar with it.
Doom the Roguelike after some legal actions known as DRL, is a top down perspective roguelike inspired by original Doom. Rip and tear all the way to the Final Evil.
Dwarf Fotress is the ultimate autism s(t)imulator. You can play it either as roguelike construction and management game or hack&slash exploration game. The depth and detail of system are without peer in the gaming world, but learning curve is steep. Original ASCII graphics can be replaced with fan made tile sets.
Enter the Gungeon is a gunfight dungeon crawler following a band of misfits seeking to shoot, loot, dodge roll and table-flip their way to personal absolution by reaching the legendary Gungeon’s ultimate treasure: the gun that can kill the past.
I didn't look at their networking, just at what it would take to put some homebrew content into their unit builder. The UI is kludged to hell. You can write JAVA UI and have it not be that, they didn't .