Question: What type of activity does Grimm and D20 System have in common?

Context articles: Advanced d20 Magic
D20 System
Castles &amp; Crusades
Engel (role-playing game)
Grimm (role-playing game)
Etherscope
Iron Heroes
D20 Modern
3d20 system
Arcana Unearthed

Advanced d20 Magic is an alternative spellcasting system for the "Dungeons and Dragons" role-playing game and the d20 System role-playing game.
 It is published by Guardians of Order, the producers of the Big Eyes Small Mouth game and the Slayers d20 game.
 The system presents a DC-based Magic System similar to the Sovereign Stone game published by Sovereign Press.
The d20 System is a role-playing game system published in 2000 by Wizards of the Coast originally developed for the third edition of "Dungeons & Dragons".
 The system is named after the 20-sided dice which are central to the core mechanics of many actions in the game.
Castles & Crusades (C&C) is a fantasy role-playing game published in 2004 by Troll Lord Games based upon a stripped-down variant of the d20 System by Wizards of the Coast.
 The game system is designed to emulate the play of earlier editions of the "Dungeons & Dragons" game while keeping the unified mechanics of the d20 System.
Engel (German for "angel" or "angels") is a role-playing game.
 The original German version uses a rule system in which the Gamemaster and players draw associative, tarot-like cards instead of rolling dice to determine the outcome of an event (this system is called the Arcana system in the original German version).
 The English version of "Engel" does not include the Arcana system at all; it is based on the d20 System instead.
 The German version includes d20 rules in addition to the Arcana system.
 The game was published by the German publisher Feder&Schwert (translates: "Feather&Sword"), who also published the World of Darkness RPG series in Germany and are now publishing the Dungeons & Dragons series.
Grimm is a role-playing game, released by Fantasy Flight Games.
 The current version is a standalone game using the Linear D6 system.
 The Linear D6 version is a significant expansion of the original material, including a more fully developed setting, a monster manual and game mechanics, all of which were previously provided by the core D20 System rule books.
Etherscope (published by Goodman Games) is a steampunk role-playing game based on the d20 system.
 It uses the "d20 Modern" rules.
Iron Heroes (previously also known as Monte Cook Presents: Iron Heroes; originally announced as Iron Lore) is a variant Player's Handbook for the d20 System of role-playing games, in the tradition of "Arcana Unearthed".
 It incorporates the core d20 mechanic, while introducing new rules options, differently-balanced classes, an entirely new feat system, and changes to the way skills and feats operate.
 It is intended to allow players to take on traditional challenges at the same levels that their D&D counterparts would be able to, without recourse to magic items or high spellcasting.
 It was written by Mike Mearls, is currently owned and updated by Adam Windsor, and published by Fiery Dragon Productions.
d20 Modern is a modern fantasy role-playing game designed by Bill Slavicsek, Jeff Grubb, Rich Redman, and Charles Ryan.
 It was published by Wizards of the Coast and released in November 1, 2002.
 The game uses the d20 System and Dungeons and Dragons 3.5 edition rules.
 It also provided players the tools to build a campaign in a modern setting.
The 3d20 system is the role-playing game system used in "Neuroshima" and "Monastyr".
 Like the d20 System, it uses twenty-sided dice, but unlike that system it most typically uses three.
Arcana Unearthed (properly Monte Cook's Arcana Unearthed, ISBN  ) is a role-playing game created by Monte Cook and first published in 2003.
 Described as a "variant player's handbook", the 256-page hardcover core rulebook bears many similarities to the "Players Handbook" of 3rd Edition "Dungeons & Dragons", on which Cook worked a few years prior.
 In terms of rules, "Arcana Unearthed" is based on the d20 system of "Dungeons & Dragons", although due to including rules for character advancement it cannot be an official d20 system product.
 (It does, however, use the Open Gaming License.)