Minetest - it's a good game, based on Minecraft idea. I love it, because it have Officila mod's/textures/and game's database, and you can install it directly from game, update, and play. It's game good for you, and you friends, you can play together on Linux, Windows, Android, *BSD, and MacOS, no limit's, like in Minecraft with Java and Bedrock edition. If you want to play minigames, it's game have some public servers with minigames, i recommend you this server. Game don't have habitual for Minecraft-player item's, like mobs, or redstone, but you can add it with mod's, or find server with pack of mod's. And last reason to play - GNU is recommend Minetest against Minecraft. If you just want to build something or survive, it's perfect game for you!
I recommend you to install Mineclone 2 or Mineclone 5, if you want to play Minecraft-like game.
You also can try Minetest in WebBrowser
Also, i found mod, that add item's, like in IndustrialCraft mod, it's my favorite Minecraft mod, and I want to share it with you: Technic, i recommend you to try Technic Plus firstly, because it's have more cool thing's.
You also can read my article called "Minetest - The Best Videogame"
It's also good and fun game. You can also get this game in Steam (Search yourself, i don't trust DRM-system) and GOG.com for free.
It's my the most favorite video-game series ever, when i'll be young, i usually play in Command and Conquer game's, and i recommend you to play Now it's beta test of Tiberian Sun. It's cool, because game is not stop on pack of three games, and expand it with time.
The best alternative to Counter Strike. And, it's fun to play with games, my friend in life, that complete a many good games said, that is cool game, and, it's not hard game, i don't play shooters about 2 years, and i don't dead usually, in that game also easier to make bunnyhop, and, this game take some part's from Team Fortress, like medic's, CTF. Game don't have imbalance gun's, and also i see sometimes map's from Counter Strike in this game, maybe it's can be full replace for Counter Strike, becuase first version's is too old for today's computer's, and new version is have soo much thing's, like casino, skin's, and DRM, you also can play with bot's.
A fun game, have some car's (As i understand, every car have unique characteristic), map's, and possability to play with friend's (Only on PC version).
Bundle of games, that help you to train you brain.
Match-Three game, like Gweled for mobile devices, a simple game, that can help you relax.
And one more game for smartphone, that help you relax, draw a beautiful sketches, and show them to you friend's, i recommend you to try, if you have smartphone.
On Internet Archive you can found a big collection of source's, and asset's of classic games, like Duck Nuckem, Quake, Doom, and many other games. You just need to run this from custon engine, like GZDoom for Doom, JFDuke for Duke Nuckem and other.
Maybe, you know about GOG.com, and it's good, but don't have bad sides, like need to complete Google Capcha. But when you complete registration, it's don't need more. And about Opensource- some time ago, i find Minigalaxy [Github mirror], opensource client for Linux, i reccomend to use it.