Sigh URLs plox cuz people are lazy.http://xonotic.org/ http://wesnoth.org/
no idea what the last pic is from though.
Xonotic in my opinion is the best arena shoot out there, but there are other good ones, such as alien arena or red eclipse, i dont have the addresses handy right now.
Wesnoth is also great, there is even an android port, but turn based strategy isnt my thing, so i cant say too much in detail about it, it certainly isnt bad.
There are also many must plays real time strategy games, i will only present you the best ones since there are so many, these are the must plays so dont miss out.
The best first: the spring engine http://springrts.com
with is many games is reachable from http://springlobby.info
, its a bit tricky to get the hang of it at first, but well worth the effort.
My personal recommendations are spring 1944 http://spring1944.github.io/
, metal factions http://metalfactions.pt/
and the cursed https://springrts.com/phpbb/viewtopic.php?f=38&p=581584#p581584
All games, maps and the spring engine are downloaded trough the lobby if you join a battle, so you dont have to do anything in most cases.
Spring and its games have a steep learning curve, but its worth the trouble.
is a age of empires like game in 3d, it has many factions set in ancient times with interesting warfare and economy.
The games is at quality is of very high quality.
is another cool game, a remake of the old C&C games (which are free too now but not FOSS BTW), it has some fun missions for in between when you have 15 minutes to kill, or you can play it in multiplayer or skirmish.
Warzone 2100 http://wz2100.net/
is another really great rts, the campaign is a must play, but dont forget to download the campaign videos and save often, also recycle obsolete units.
Another cool game which isnt a RTS but a FPS/RTS hybrid is tremulous, it is available with a new client by grangerhub http://grangerhub.com
This is a real piece of cult FOSS game with a very rich and incredibly interesting history, players and selection of maps.
Be sure to give it a shot.
I also made this poll at http://www.strawpoll.me/12737591
to see what games people here would want to play, if there is enough interest we can work something out.
There are many many more good FOSS games, finding them all is a hard piece of work, but i will keep you guys on the loop as much as i can.
I will post more later on as i verify some more games.