Free Game Engines

A game engine is a piece of software intended for game developers. It is almost a complete game, just add content. There are advantages to using a game engine to write your game. It is probably the simplest way to tell a story, next to writing a book. You don't have to be able to program to use an engine. Scripting is usually optional. You can finish a game much faster by starting with a pre-built engine, rather than writing your own from scratch. A game that starts out with a pre-built engine is much more likely to actually get done.

There are disadvantages too. Using a pre-built engine is much less flexible than writing your own. And you have to obey whatever license you are allowed to distribute the engine under.

A programming library is a piece of software that helps programmers, by implementing common functions so that they don't have to. You have to know how to program to use a library. Using a library to program your own engine allows you much more flexibility than using a pre-built engine. SDL, Crystal Space, Love2d and ODE are libraries that you might want to use to write your own engine, if that is the route you prefer.

You can get hundreds of game engines from the internet. Some of them are free. You can do anything you want with these, including selling them alongside your content for a fee. Most of the game engines on the internet, however, are not free. Most of them are closed-source. Often, you have to pay the creators a fee if you use their engine in a commercial product. Sometimes, a game engine is tied to a particular service, and your game can never stop using their service.

I have collected a list of complete and functional free game engines. You can use these engines without restriction. Please note, however, that there are obligations with certain licenses. For example, with the GPL, you have to include the source code to the engine, or make a written offer to deliver it. Make a point to read the full license of any engines before commiting to using them.

If you have a game engine you would like to recommend for this list, please contact me.

Gamebook

Commercial examples: Choose Your Own Adventure Series, Lone Wolf Series, Fighting Fantasy Series
Free Engine: The Gamebook Engine *
License: GPLv2
Pros: 2 complete games. book format is simple and flexible.
Cons: graphic support is rudimentary. New. Example gamebooks are not free.

Real Time Strategy Game (RTS)

Commercial examples: Starcraft Series, Warcraft I, II and III, Age of Empires
Free Engine: Bos-Wars Engine
License: GPLv2
Pros: has a campaign
Cons: poor documentation for creating maps or manipulating the GUI, maps slow
down a lot while playing, AI plays poorly, has an unfinished feel
Free Engine: Stratagus
License: GPLv2
Pros: has a few complete games using it.
Cons: development has "permanently halted." The team is now working on the
Bos-Wars Engine, which is a modified version of Stratagus.

Turn Based Strategy Game

Commercial Examples: Warlord, Lords of Magic
Free Engine: Battle for Wesnoth
License: GPLv2
Pros: popular, large and active community, and many complete maps/campaigns.
Cons: Maps use a proprietary, verbose metaformat. Scripting is hacked in, and
difficult to write. Documentation is a clusterfuck, with no clear beginning for
learning to write maps. "Stable" releases aren't really stable until around
1.x.6.

First Person Shooter Game (FPS)

Commercial Examples: Doom Series, Quake Series, Half-Life Series
Free Engine: Dark Places
License: GPLv2
Pros:
Cons:
Free Engine: ioquake3
License: GPLv2
Pros:
Cons:

Unfinished Game Engines

These game engines are not complete. Either they don't have a complete implementation, or not everything necessary is there for you to use to make a game. If you are trying to make a game in one of these genres, I recommend checking these projects out. If you can use it, great. If you can't, you might want to help the project out, so that you can.

Massively Multiplayer Online Roleplaying Game (MMORPG)

Commercial Examples: WOW, Runescape, EVE, Ragnorak
Free Engine: The mana world, Worldforge, Planeshift
License: GPLv2, GPLv2, GPLv2
Pros:
Cons: Worldforge is underdocumented. It is difficult to figure out which files you need to
edit to do what, and how to modify them. Planeshift requires the Crystal Space game library,
which is also underdocumented. Crystal Space is not yet stable, and it's dependencies are
numerous, unstable, and rapidly changing. Planeshift does not have any free content you can
use to start your game.

Role Playing Game

Commercial Examples: Final Fantasy I, II, III, Dragon Warrior I-IV
Free Engine: KQ Lives RPG *
License: GPLv2
Pros: retro feel.
Cons: A lot of content modifications have to be done in the engine itself.
Free Engine: Hero of Allacrost *
License: GPLv2
Pros: Very nice graphics and sound. Supports everything you would expect in a
classic RPG with flexible and nearly complete scripting support.
Cons: Scripting language is verbose and rudimentary. Still in Beta. Not
officially advertised as a generic RPG engine, and you might not receive support
for using it in your game.

Isometric Adventure Game

Commercial Examples: Fallout I, II, Diablo I, II
Free Engine: Flexible Isometric Fallout-like Engine
License: LGPLv2.1
Pros:
Cons: No working game demo. New.
* I am a developer for this game engine.