Lugaru HD was a part of the Humble Indie Bundle which allowed users to pay whatever the want for some great cross-platform games.

Alien Arena is a free, stand-alone FPS deathmatch game for Windows and Linux.

Zaz is an multi-platform (Windows and Linux) arcade action puzzle game where the objective is to get rid of all incoming balls by rearranging their order. The games seems really addictive, so be aware! :-)

Battle for Wesnoth

Nexuiz is one of the most popular shooter games which emerged after the open-sourcing of the Quake 3 engine, featuring a fast-paced game style and several game modes, like the popular DM or CTF.

Heroes of Newerth (or HoN) requires an invite to play, but it's developers created an HoN Holiday Open Registration form so you can sign-up for Heroes of Newerth for free - for the next 83 hours

Aquaria is an action-adventure single-player game by independent game company Bit Blot. The game is commercial and runs on Windows and Mac but as we don't usually write about commercial stuff, I will only present the Linux native version which is in open beta and is free until January (full version).

Commander Stalin is a free, open source real-time strategy based on Boswars game for Windows and Linux.

QuantZ, is a "unique puzzle action game" based on physics, where your goal will be to control a three-dimensional object positioned at the center of the screen. Balls that gravitate around can be eliminated by shooting them into balls of the same color.

Tremulous is a free, open source game for Windows, Linux and MacOSX (unofficial version for Mac OSX) based on the ioquake3 game engine, that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans.

PlayOnLinux is a free application that puts an easy to follow wizard style interface on top of WINE and contains some pre-installed scripts to help load a number of compatible applications and games.

D2X-XL is an OpenGL mod for the classic 3D game Descent 2. D2X-XL is based on D2X 0.2.6, has all bugs from D2X 0.2.6 fixed and has restored all features from the original D2_3dfx (official Voodoo graphics version). D2X-XL is free and runs on Windows, Linux and Mac OSX.

Smokin' Guns is a free, open source first person shooter game for Windows and Linux that takes you back to the times of sheriffs and bank robbers in the legendary Wild West.