Battle for Wesnoth
Battle for Wesnoth, or BfW for short, is a popular turn-based strategy game which takes place in a fantasy universe and has support for singleplayer and multiplayer, official campaigns (and more available to download), hotseat games. The game comes by default with 6 factions and takes place over hexagons, each player deploying his army and trying to kill his opponent. Wesnoth can be highly modded via WML (Wesnoth Markup Language), and the add-ons server includes many more maps, factions, eras and campaigns. Beside for the usual mode which allows up to 9 players to battle against each other or forming teams, there are also the rumble maps (very small maps), or the survival ones, or the multiplayer campaigns or role playing maps. There is also an unofficial ladder available. Wesnoth is really an awesome, complete game, with a great community, great online playing, music themes, map editor, a great helping system, and much, much more.

sudo apt-get install wesnoth

AssaultCube
Being based on the Cube engine, AssaultCube is a very fast-paced first-person shooter with a gameplay that changes from mode to mode. There are the usual TDM or CTF modes, but also TOSOK (Team One Shot One Kill), KTF (Keep the Flag), OSOK, LSS (Last Swiss Standing) or even PF (Pistol Frenzy). It doesn’t have high system requirements and the online servers are most of the time filled with players.

sudo apt-get install assaultcube

Neverball
Neverball is a 3D action where you control a ball and you have to collect coins. You can start with a beginner level and advance to more advanced levels. Although the concept seems pretty simple, the game is not easy to master and it becomes addictive even in the first minutes of playing. Neverball allows configuration of sound, resolution, graphics details. It can be played in fullscreen mode too, and you can choose from various ball models.

sudo apt-get install neverball

Xonotic
Xonotic is based on Nexuiz, the result of a fork which aims to keep it free and open-source. Xonotic is a shooter with a fast game style, single player and multiplayer modes, lots of maps, models and weapons to choose from.

Download

Hedgewars
This is a Worms-like game, completely free and open-source, which keeps the funny atmosphere the original Worms had. Players control the so-called “hedgehogs”, which take turns, and they have entire arsenals at their disposal to blow their enemies off the orbit. A very funny and addictive game.

sudo apt-get install hedgewars

Warzone 2100
Warzone 2100 is a real time strategy game set in a future, apocalyptic universe, and it provides single player mode, multiplayer, campaigns and lots of maps. The true thing that sets Warzone 2100 apart is the unit/building tech tree, which can be upgraded differently using over 400 technologies.

sudo apt-get install warzone2100