The d20 system is a Game System for Tabletop RPGs created by Wizards of the Coast and premiering as the system used for the third edition of Dungeons & Dragons. What makes the d20 system unique is that it was released under the Open Game License (OGL), which was created by Wizards specifically for that purpose. Under the terms of the OGL, developers are granted permission to copy, use, modify, and redistribute the system — effectively making it analogous to open source software. This means that anyone can create their own RPG using the d20 system, and even sell it for profit, without running afoul of copyright issues.
The core mechanic of the d20 system is rolling a twenty-sided die (the eponymous d20), adding a "modifier" to the result, and comparing that to a "difficulty class" (DC) to determine whether the action was successful or not. The d20 provides randomness, the modifier represents the character's skill, and the DC represents the difficulty of the action being attempted. If the total result (d20 + modifier) is greater than the DC, then the action succeeds. Modifiers are dependent on the character's stats, and both modifier and DC may be altered by situational bonuses or penalties (for example, if your target is lying prone, ranged attacks become harder, but melee attacks become easier). The most important modifiers are generally The Six Stats, at least one of which will be applied to almost every roll, but other check-specific stats will also be included. Also of note is the "opposed check" — when two characters directly oppose each other (one character tries to sneak past another, for example), then both will make the relevant skill check (eg, stealth vs. notice), and the higher result wins.
Outside of this core mechanic, the system is very flexible, and include everything from the classic Class and Level System of D&D to the Point Build System of Mutants & Masterminds. This flexibility is a large part of the reason for the system's popularity — it allows different systems to cater to different tastes, without really changing how the game is played, as "d20 + modifiers vs. DC" remains constant.
For a copy of the revised System Reference Document (SRD v3.5), you can find the original here.
Games that use the d20 system:
- Dungeons & Dragons (3rd Edition). The d20 system was created by Wizards of the Coast for this edition, although later editions have substantially revised upon it.
- Arcana Unearthed/Arcana Evolved by Malhavoc Press is a major rewrite with new spells, new classes, and new types of magic. Generally highly praised.
- d20 Modern and d20 Future, both developed by Wizards of the Coast, were attempts to bring the d20 system out of the Heroic Fantasy setting iconic to D&D proper.
- d20 Rebirth is a fan-made system in development, built along the same logic as Pathfinder.
- Iron Heroes, a system from Fiery Dragon Productions, is dedicated to more tactical fighting than vanilla 3e D&D. The game itself borrows heavily from action hero and fantasy movie tropes.
- The Legend System actually began as a series of house rules for 3.5 before the writers decided they had enough material for a full game.
- Microlite20 is a free, extremely streamlined and rules-lite version of the d20 system, designed to be compatible with existing d20 monsters and adventure modules.
- Mutants & Masterminds, a superhero-themed Point Buy RPG, using the d20 system for its core mechanics.
- Northern Crown, a North American fantasy setting by Atlas Games.
- Pathfinder by Paizo Publishing is notable for being made specifically in response to Wizards building a new Dungeons & Dragons edition. Pathfinder is written to be compatible with D&D v 3.5, and fix some of the remaining flaws in the system, though the quality of its changes is a contentious subject among fans. Successor to the 3.5 throne, it is highly successful commercially and widely supported by third parties.
- Pathfinder Society is the ongoing public campaign run by Paizo using the Pathfinder system; it spans an overarching plot of several seasons.
- 13th Age uses a variant of the d20 system that draws in some elements of 4th Edition D&D, which is to be expected considering the co-designers are Jonathan Tweet (one of the lead designers of 3rd Edition and the d20 system) and Rob Heinsoo (one of the lead designers of 4th Edition. Also has elements of storytelling systems, such as powerful icons in the setting, and skills based on the characters' backgrounds.
- True 20 is a simplified iteration of 3.5 from Green Ronin Publishing. Adapted to a wider variety of genres than D&D, it's a Spiritual Successor of sorts to d20 Modern. Its major selling point is ease of use and speed of play.
- A large number of unrelated systems and/or settings have d20 variants: