Not enough love for Greg Stolze in this thread. He's the best person in the role playing field. Not even joking. I suggest you all go look him, and his vast works, up on your own. I'll give you the broad strokes here.
First: He made Unknown Armies, which is like if White Wolf's system was cool. The setting has magic and such, but to perform magic you need to adhere to particular lifestyles. You might accumulate mojo by hoarding books, or porn, or always winning arguments no matter how little, or being the best salesman, or personifying the concept of 'the lone wolf', or whatever. Of course, your character doesn't do these things because of the magic it grants them. No, your character is so strongly attuned to these ideals that it is their worldview. That is who they are. And they are that way so bad that the world molds to fit them. You have to be insane to be a powerful magician. Someone once described being magic in Unknown Armies as "having the keys to a sleek, sexy, souped-up sports car while everyone else is driving clunkers- but it only goes full speed and into a brick wall."
Second: He helped extensively with One Role Engine (ORE), which is the default system for Reign, Wild Talents, Progenitor, etc. The Progenitor setting, in particular, is the most in-depth and well constructed alternate-history scenario of anything ever- essentially, a random housewife in the 1960s becomes the focus of .05% of the universe's dark energy, turning her into a demi(?)-goddess. She joins the US war effort to settle things over in Vietnam. It is only later that it is realized her power is contagious, and those she uses them on in turn begin manifesting abilities of their own. And on, and on, until 1999, where the setting's official timeline ends, leaving you a world full of superpowered pimps and presidents and supergenius hippies.
The system of ORE though, is amusing, because it is intuitive for even a systems-dumb person like me to get into. You add your stat score and skill/power score together to form a dice pool. You then roll that many d10s. You then find matches. So if you have a dice pool of 5, and you roll a 1, 2, 3, 3, 4, you have one 'set', the two threes. This is a 2x3. 2 being the width, and 3 being the height. Larger dice pools mean you have a chance to get better sets, and you can mix and match. You can spend more points on your stats and skills to get 'Hard Dice' which always come up 10, and 'Wiggle Dice', which are wildcards. That is basically all you need to know to 'get' the system.
I basically can't express enough how radical Stolze and everything he does is.