Open Source Games
The Doom, Quake, and Quake 2 engines are fully open-source. The data files aren't but you should have those anyway. The engines can be used to build your own games, to fix bugs & problems, or to port the game to new & exciting systems. The Quake source code has been used to make ultra-high-detail versions of the game with light sourcing, shadows, and scary monsters. Doom has been ported to everything from mobile phone to obscure Korean handheld consoles. Fully 3D versions of Doom with shadows and everything have been made using the released sources. On the whole, a hacker with some source code can get very creative!
Lucasarts' classic Monkey Island series works great under Linux thanks to ScummVM
However, not all companies are forthcoming with old source. The old Lucasarts point & click games, such as Monkey Island, have no source available for people to read through. That doesn't stop the average hacker though - and thus reverse-engineering of games takes place. By analyzing the data files for games, developers can work out the missing bit (i.e. the engine). The ScummVM project is one example, which can play all Lucasarts point and click games, and a few besides. The work was impressive enough that other point & click companies helped get their games working properly - Broken Sword now runs in ScummVM, and Beneath a Steel Sky has been released as freeware to be played with ScummVM.
Not all open-source game efforts are related to the big commercial works though. Some really creative independent work goes on - Tux Racer is the best known example, a simple game where you guide the Linux mascot down a mountain at high speed collecting sufficient fish and passing the line fast enough to pass the course. It is by no means the only one though. Neverball, for example, is a good fun game similar in style to Super Monkey Ball for Nintendo Gamecube. You use the mouse to guide your ball around fiendish courses, collecting enough coins to open the exit. Egoboo is a 3D hack & slash game based in part on old text-console game Nethack.
Neverball - a game modelled after Super Monkey Ball - for Linux, Mac and Windows
Whilst these open-source games are a testament to what the open source process can achieve, not everybody wants to be playing KMahjongg rather than Half Life 2. And, luddites that they are, there are still many companies which don't make games for Linux. At this point, a little extra help is needed for the neophyte Linux gamer.
