Open Game Systems
This page collects games released under an open license. Generally speaking, if something is available under an open license it is free for use and re-use as long as the terms of its license are respected, without any explicit contact or negotiation between original author and licensee. Requirements vary from offering no restriction whatsoever to requiring that you credit the authors or that you also allow others to use your own derivative work freely.
The most common use of these licenses is for the original authors to inform other people interested in using their system that they are legally able to do so and even sell their own new work without having to pay anything to the original authors - however, even if those are intended as a gesture of good will and openness, failing to respect all of the terms from a license may leave you exposed to a lawsuit from the original creators.
Using an openly-licensed system for your own products (books, video games, card games...) means that you are able to rely on the work of others instead of having to design, write, playtest and balance your own. It also allows a product to become part of a larger community - which may draw those players (since they're already familiar with the rules) or allow them to combine your product with others that use the same system.
Here is a brief description of each major license found on the list below. Note, however, that details vary from license to license - so if you're planning on doing anything more than using the systems below for personal play, you should get acquainted with the full text of the license:
- Open Game License (OGL): the OGL allows for differentiation between Open Game Content (parts of the system that others can use freely) and Product Identity (parts that are not open for others to use). Some publishers also create a System Reference Document (SRD), which is a version of the system containing only the Open Game Content, without any Product Identity.
- Creative commons (CC): a family of permissive licenses that usually require only that you credit the original authors when using their work. Authors are free to choose from any of the CC licenses available - to read more about the differences between each, visit their official website.
- GNU licenses (GPL and GFDL): designed for distribution of computer code and programs, this license allows you to use original work freely. However, if you are creating a modification of said work (for example, a set of alternative rules), you are required to make your own work available through the GPL license as well. If you are not modifying the original work, but only using it to create something entirely new (for example, a book with new characters and monsters to use) then you are not required to distribute it under the GPL. There are many versions of this license available, the most popular arguably being the GPLv2 - for more information about each version and related licenses see the official GNU website.
Some open games, despite being free to use and modify in derived work, require you to buy a printed book or PDF copy to learn the rules. The list below should only contain examples of systems that are also 100% free to acquire digitally - or at least allow you to look up most of the rules online.
Open Game License
- Original OGL
- Dungeons and Dragons 3.5 SRD
- Hypertext D&D 3.5 SRD
- Pathfinder SRD
- d20 Modern SRD
- Dungeons and Dragons 5 
Note that the official d20 SRD for D&D 3e has been revoked. It has been replaced by Paizo's Pathfinder SRD, which is compatible with previous d20 material and still being used for publication of new products by many companies and the community at large.
- d20 Anime SRD (direct link to file)
- Wayfarers (OGL and open-ended agreement)
- Prometheus SRD (OGL and Prometheus License)
- Mutants & Masterminds (OGL and M&M Superlink)
- True20 (OGL and True20 Adventure Roleplaying Trademark License)
Retro-clones are new games that are compatible with older games now out-of-print, allowing for the use of previous material without having to acquire a possibly rare and/or expensive copy of the original game or resorting to piracy. These are part of a larger movement known as the Old-School Renaissance (OSR).
- OSRIC for 1E AD&D (download) (OGL and OSRIC Open License)
- Labyrinth Lord for B/X D&D (download or view wiki) (OGL and Labyrinth Lord Trademark License)
- BFRPG, Basic Fantasy Role-Playing Game, for B/X D&D
- Swords & Wizardry for 0E D&D (download) (OGL and the Swords & Wizardry Compatability Statement License)
- Swords & Wizardry Whitebox for Whitebox D&D (download) (OGL and the Swords & Wizardry Compatability Statement License)
- Warrior & Wizard for The Fantasy Trip (download CC or OGL version or view wiki) (Creative Commons Attribution-ShareAlike and OGL; Direct link to file)
OGL with trademark license
These games release their material as Open Game Content but also provide another, more restricted license to allow publishers to refer to Product Identity.
- Mutant Future (OGL and Mutant Future Trademark License)
- GORE (OGL and GORE License)
- FUDGE (OGL and FUDGE System Trademark License; also under its own license)
- vsM Engine
Creative Commons licenses
- 6d6 RPG 6d6 Fireball (Attribution-Share Alike)
- Eclipse Phase Posthuman Studios (Attribution-Noncommercial-Share Alike 3.0 Unported License)
- Drowning and Falling (Attribution/Non-Commercial/Share-Alike)
- The Shadow of Yesterday (Attribution)
- Donjon (Attribution/Share-Alike)
- Wushu Open (Attribution)
- Wushu Open Reloaded (Attribution/Share-Alike)
- Shadows (Attribution)
- Myriad (Attribution)
- Organic Rules Components (ORC) (Attribution)
- Violence (Attribution/Non-Commercial/Share-Alike)
- Star Wreck (Attribution/Non-Commercial/Share-Alike)
- Noteworthy (Attribution/Non-Commercial)
- Executive Decision (Attribution/Non-Commercial/Share-Alike; direct link to file)
- ...in Spaaace! (Attribution/Non-Commercial/Share-Alike; direct link to file)
- Sufficiently Advanced (Attribution/Non-Commercial)
- Treasure (Attribution/Non-Commercial/Share-Alike)
- The Simple Game System (TSGS) (Attribution/SHare-Alike)
- Danger Patrol (Attribution/Non-Commercial/Share-Alike)
- Lady Blackbird (Attribution Non-Commercial Share-Alike)
- Iliamna Unknown (Attribution Non-Commercial Share-Alike)
- Saga, a rules-light narrative universal system (Attribution/Non-Commercial/Share-Alike)
- Jumpers, a multi-dimensional, multi-genre sci-fi RPG (Attribution-Noncommercial-Share Alike 3.0 Unported License)
- Gods and Monsters (GNU Free Documentation License)
- Circe (GNU Free Documentation Licence and GNU General Public License)
- Fringe (OpenOffice Writer format) (GNU General Public License)
- Dominion Rules (Dominion Rules Licence)
- Action! System (Action! System Open License)
- Active Exploits (Active Exploits Distribution and Expansion License)
- Redbox Hack (view wiki) (license unclear; readers told to "do what you'd like with the thing").
- Dungeons and Dragons Fourth Edition (Game System License)
- Fire and Sword 4E (Free Art License)
- 4C System for the Marvel RPG (download) (Public Domain; Direct link to file)
- The Far Towers (Public Domain)
- Hoard 1st Edition and Microcosm (Public Domain)
- Theonosis, an open fantasy setting - The Open Setting License allows the setting to be used with any game.
- List of open tabletop games at the FOSsil Bank wiki.