Mouse Guard, by Luke Crane, and Apocalypse World, by D. Vincent Baker, seem to have more than their fair share of hacks. While there are lots of OGL games derived from the d20 system and Fate, I'm excluding those for the time being.
Mouse Guard has Realm Guard, Mouse Run, Song of Fire and Ice Hack, and others from the hack message board.
Apocalypse World has the Lovecraft World, the full-blown Dungeon World, and others from the message board.
What Makes them So Hackable?
First and foremost, Luke Crane and D. Vincent Baker are at the top of their game. They are both highly regarded game designers. Their works are well defined and tightly designed.
Mouse Guard is a game about small frail mice fighting, against their nature, for what they believe in. The system is a distilled version of Burning Wheel, infused with the essence of David Petersen‘s “Mouse Guard” comic.
Apocalypse World is a game about fragile humans fighting to survive and hopefully thrive in a hostile world not of their making. The core of the system is very simple, but D. Vincent Baker masterfully applies layers to the core, crafting an inspiration game system.
Both of the above systems provide a fundamental conflict resolution and then masterfully decorate that mechanism with the essence of their design.
In Mouse Guard, there are a handful of questions that you answer to slowly build up your mouse. Once you are done, you have relationships, a few odd skills (i.e. Apiarist, Weaver, etc) and some core skills for being a member of the Mouse Guard.
In Apocalypse World, you choose your Playbook (i.e. archetype), and check off a few boxes to customize your character, then establish your relation to the other characters.
Both systems are very quick on character creation, yet focus on giving your character a place in the world. The skills of Mouse Guard are easily exchanged for different ones (hence the hackability). The playbooks of Apocalypse World are meant to be hacked
In Mouse Guard, the resolution mechanics are roll some dice against a target obstacle. You can get bonus dice from those willing to help; Or by FoRKing your own skill into the test; Or by tapping your mouse-like Nature; Or by spending Artha.
In Apocalypse World, the resolution mechanic is roll 2d6, and add some small modifier (-2 to +3); On a 10+ you get what you want; On a 7 to 9 you get what you want at a cost; On a 6 or less the Master of Ceremonies (i.e. GM) gets to make a move.
Personally, I believe the fundamental component of Mouse Guard is Nature. Much like Burning Wheel's Faith, Hatred, Greed, and Sorrow, it is a mouse's nature that defines him.
As such, any hack of Mouse Guard must first and foremost consider what does a character's Nature represent, and it needs to run contrary to what the character would normally be attempting.
In other words, a mouse's nature is to run, hide, and climb; Those are contrary to being a member of the Mouse Guard.
The fundamentals of Apocalypse World, on the other hand are encoded in the moves; Both MC and Player moves. In customizing the moves, you define the nature of the game.
Mouse Guard and Apocalypse World set out to create a particular game, and do it quite well. In doing so, however, the designers have each created an excellent core framework. With a bit of elbow grease, and plenty of posts to forums, a hack can be created.