Find a retro-clone of Gamma World, such as Mutant Future. PDF's for retro-clones generally follow the open-license model and you can find them for free.
Being that this is a Fallout forum I cannot see how you can do less, as I'm almost certain it's an inspiration for Fallout. It hews very heavily to oldschool gaming sensibilities where you roll for almost everything and let the dice fall where they may. Mutant characters generally roll for random mutations and you can wind up with either beneficial or harmful mutations -- or develop new ones through radiation. The setting is camp but it still assumes that pre-war ruins exist in logical places and that mutant lifeforms have largely inherited the earth.
Incidentally, Adventure Time draws very heavily from D&D and Gamma World.
I'd avoid the New World of Darkness (nWoD) as they do definitely feel dumbed-down for mass appeal -- lore included.
Find Vampire the Masquerade for the non-reboot World of Darkness, either one of the older rulebooks or White Wolf's anniversary reprint. There's a lot I find interesting about their Storyteller System itself. I generally like their idea of Backgrounds and the basic idea of Willpower tokens, although the rest of the system follows the general formula of skill check + attribute check. Not all of it is exactly original anymore though but it's an example of a skill-based system with perks, morality meters and other doodads bolted on. Again, nWoD dumbs a lot of this stuff down, as I'm under the impression that skill checks could be curved more easily in oWoD.
For example, Willpower gets used in other systems as "plot points" or "Fate" or "Luck" or something similar and effectively act as chits that let players boost their character's performance or bend plot elements in their favor in a major or minor way. And it can be a way to incentivize the player to behave in certain ways. In particular, oWoD lets players replenish Willpower whenever they succeed at a task that aligns with their personality type. And it can be used to resist primal instincts such a vampire's lust for blood or his fear of fire/sunlight. A particularly important task, since the "default" playstyle assumes that PC's want to retain their Humanity. PC's become increasingly feral and more visibly monstrous as they lose Humanity. (Other vampire factions can be bizarrely alien and no longer cling to any vestige of conventional morality.)
The setting itself is rather deep though and the Vampire the Masquerade cRPG is rather excellent (if buggy) and does an incredible job of showing off the lore of the setting. It still impresses me how much they were able to cram into it.
The politics and factions of the Vampire universe are what makes the setting so damn interesting. It shares the universe with the rest of the WoD supernaturals but it's always interesting to see it from the vampire's perspective. Vampires generally fear Werewolves as being nigh-unstoppable killing machines who absolutely loathe their kind for reasons which are unfathomably alien to them. Then you can pick up the oWoD Werewolf book and find out exactly what makes the average Garou tick and why they hate Vampires.
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