I wrote some articles a few years ago, but they are still useful today:
These are great resources, thanks!
Many on F-Droid.
If we're talking stuff on F-Droid, the big one there for me is UnCiv. It's an excellent fully-free reimplementation of Civ V... with all the nightmarish one-more-turn-oh-God-is-it-dawn addictive problems that implies. Only real flaw is that by adapting Civ V, it also adapts Civ V's big flaw: traffic jams. Unciv units neither stack nor combine so waging war in an obstacle-rich landscape is hellishly tedious. Also the higher difficulties feel just abusively random and unfair because the hard-level AIs get free resources, but that's normal for a Civ game.
Shattered Pixel Dungeon
Not all are open source, but a fair chunk are. All games on the list are free or one time payments.
Zero-K. It's a competitive online RTS loosely based on the classic Total Annihilation (which led to Planetary Annihilation and Supreme Commander). Some of its features are a bit overcomplicated, but it does an amazing job innovating within the RTS genre.
It's fast, aggressive, and fun. You spam units, claim territory (in the form of metal-extractors and energy-grid that upgrades their output, and building defenses to protect that) and raid and assault your opponents.
It abandons the hoary old concept of factions, instead giving you your choice of starting factories... and as the game progresses, you can expand into other factories to access the synergy of units. So you start a battle with a narrow slice of the unit-pie, but ultimately can access the whole inventory in a single match. There is no "teching" really, besides constructing resource-buildings, which keeps the focus on resources, construction, and combat.
It has a full single-player campaign that introduces the game's complexity bit-by-bit... but the campaign does have some difficulty spikes, particularly since the units do get rebalanced once in a while and so an old mission will become suddenly easier or harder as the developers patch the game.
The game has a Lua-based GUI plug-in architecture if you like WoW-style UI mods as well.
Xonotic;
The Battle for Wesnoth;
and yes, Veloren is awesome.
Endless Sky is great
@popcar2 Looked over the other variations of this thread and didn't see Prospector mentioned: http://www.prospector.at/forum/about.php
It's a scifi roguelike where you lead a team of prospectors to try to recover valuables from across space to make enough money to retire.
Kind of in an odd spot source-wise, as the recent source code technically isn't open/available (last open releases were 10+ years ago), so it may no longer really fit, but seemed worth mentioning nevertheless.
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