There are plenty of excellent games on Linux, and a fair amount of them are completely free. Some are open source, and others are fairly big names available through Steam. In every case, these are quality games that you can play any time on Linux at absolutely no cost.
DoTA 2DoTA 2 is one of Valve's biggest titles. It's been around for a fairly long time, and it was one of the first games Valve ported to Linux when they started supporting the OS with Steam. It was also one of the first Linux games to receive Vulkan support.
DoTA 2 is one of the biggest MoBA games and a giant player in the eSports space. Valve continues to update DoTA 2 with new heroes and content. Because DoTA 2 is an eSports title with an active online playerbase, you're never going to run out of things to do in this game.
Team Fortress 2Team Fortress 2 is another major title from Valve. It's a cartoon first person shooter that allows you to take the role of one several characters with different abilities. Together with your team, you fight an opposing team over several objectives. Team Fortress 2 might not be as big as it once was, but it's still a very popular title with complete Linux support. With an active online community and competitive play, this one shouldn't get old for a long time.
WakfuWakfu is another excellent free game available on Steam. This is a classic anime-style turn based tactical RPG with a movement grid. Wakfu is actually a free to play MMORPG with an active and dedicated community.
If you're a fan of classic JRPGs, or you're just looking for something different, this one is a fantastic option. Because Wakfu is an MMO that's still being actively supported, more content is always being added to the game, making it a great long term favorite.
XonoticXonotic is one of those games that goes back a long way with Linux gamers. It's an open source sci-fi first person shooter with a fast paced gameplay feel that might remind you of a mix between Quake and Timesplitters.
Xonotic might not be a AAA blockbuster, but it's still a fun game with online play. Because it's open source, you might even find it in your distribution's repositories. Even if it's not, you still won't have a problem downloading it and getting running real quick.
Battle For WesnothBattle for Wesnoth is a turn based strategy game set in a medieval fantasy world. Wesnoth is open source, and it's available in many distribution repositories as well as on Steam. This is another game that's been synonymous with Linux gaming for a long time.
Wesnoth has a endearing old school vibe with pixel graphics, don't let that trick you into thinking this is a dated game. Instead, it blends nostalgic visuals with genuinely fun and engaging gameplay to make one awesome free game.
0 A.D.If real time strategy is more your thing, check out 0 A.D. It's an open source real time strategy(RTS) game that's probably available in your distribution's repositories. 0 A.D. has been popular for a long time in the Linux world, and for good reason. It's actually a great RTS game that can rival some commercial options.
0 A.D. is still under continual development, and receives regular updates. It features fully 3D gameplay and units with combat animations. Over the years, the graphics of 0 A.D. have improved dramatically, so it looks pretty great, especially for a free game.
SuperTuxKartFor years, SuperTuxKart was a joke of sorts when talking about Linux gaming. Its an open source clone of the popular Mario Kart series, but featuring Tux instead of Nintendo's trademark plumber. Still, the game is actually fun, free, and probably in your distribution's repositories. SuperTuxKart is fully 3D and will run on nearly any Linux computer, regardless of your system specs, making it a great option for standard desktops as well as gaming PCs.
The Dark ModIf you like the classic game, Thief, check out The Dark Mod. First off, it's not technically a mod, at least not anymore. It started out as a total conversion on top of Doom 3, but it's since evolved into its own standalone game that you can download directly from the developer's website.
The Dark Mode is a lot like Theif, with the player acting as a thief in a steampunk style world, but it isn't set in the same world and doesn't use any of the assets from Theif. This is it's on game, a fan created tribute of sorts. That said, it's still an actively developed title with regular updates, new content, and continual improvements. If you're a fan of stealth gameplay at all, this one should be on your to-play list.
HedgeWarsHedgeWars is a modern remake of the classic game Worms, but with its own twists and improvements. Like Worms, HedgeWars is all about strategy and choosing the right tools for each situation. HedgeWars adds a more comedic style and multiplayer.
HedgeWars is opens source, cross platform, and of course, free to play. It's under constant development with new tools, weapons, and customization options always popping up.
AwesomeNautsAwesomeNauts is another free Steam game for Linux. This one isn't from Valve. Instead, it's one of the growing number of indie titles to support Linux on Steam. AwesomeNauts is a 2D side scrolling MoBA that feels like a strange cross between DoTA 2 and Mega Man. Somehow, that works in a really great way,
While serious MoBA fans might not like the side scrolling gameplay, if you're looking for something that's just pure chaotic fun, this one is definitely a great option.