In general, Ground Zero uses rules and definitions from d20 Modern. After playing a couple adventures with the full d20 Modern rules, I decided to strip them down and simplify as much as possible. If you're new to the d20 system, check out The Basics (on this website) for a the fundamentals. You can also download the entire d20 Modern System Reference Document for free (kindly made available by WotC). If you're familiar with D&D, Call of Cthulhu d20, or d20 Star Wars you should be able to get by just fine.
Additional rules have been adapted from Dominic Covey's "Darwin's World", Jonathan Tweet's "Omega World", and Gamma World d20. A lot of material was also "borrowed" from Fantasy Flight Games' "Redline" and Monte Cook's "Chaositech". Finally, there's a fair number of my own house rules. I've tried to put enough information online to ensure that you can learn all the rules from the website.
The point "homebrewing" the rules is to streamline and simplify, wherever possible, the d20 system to create more of a "fast-and-loose" approach to gaming. Hopefully they won't confuse the hell out of everyone.
Important: Even with all the house rules in Ground Zero, most of the standard d20 rules remain the same. You still roll a d20, add your modifiers, and try to beat a target DC. But with all of the tinkering under the hood, there are a few basic changes that you should be aware of up front...
Several steps of the character creation process involve Build Points (BPs). Build Points are meant to counteract some of the randomness of character creation in Ground Zero and give the player some control over their PC. Although attributes and mutations are rolled randomly, BPs can be spent to modify or reroll some of the results. Ground Zero PCs start with 30 BPs. More information on Build Points can be found on the Making a Mutant page and elsewhere.
"Abilities" are called "Attributes" because I like the name better. Characters randomly determine their attributes, then modify them using Build Points. I find this semi-random method much more exciting than the standard rules.
Also; note that hit points a bit differently than usual. All characters receive base hit points equal to their Constitution score (plus size modifier). Constitution does not modify hit points gained for class levels. Instead of rolling randomly for hit points at each new level, characters receive a fixed number, depending on class.
Instead of race and occupation, all characters in Ground Zero must select their genotype and background. As the campaign progresses, players will have to option of choosing additional genotypes and backgrounds.
Most characters will possess several mutations; some of which will be beneficial (mutant perks), others baneful (mutant flaws). Mutations are determined randomly and are the chief way that PCs are differentiated from each other.
The character classes in Ground Zero are different than those in a standard D&D or d20 Modern game: PCs can start off as enforcers, fringers, scavengers, or thinkers. Characters in the Tribal Defenders campaign begin at 2nd level. In general, a character's class in Ground Zero is less important than his mutations and equipment.
Skills and talents in Ground Zero are significantly different than in a usual d20 game - they have been changed to allow for more of a "fast-and-loose" game style. You should read the Skills & Talents page for details.
Many individual skills have been grouped into more general selections. Climb, Jump, and Swim, for example, have been rolled into the single Athletics skill. To compensate for the lower number of skills, starting characters receive double their normal skill points at 1st level, not quadruple.
And as for talents... I always thought that "feats" was a retarded name. And the class talents from d20 Modern were pretty damn indistinguishable from feats anyways. For Ground Zero, I've rolled feats into the talent system, simplifying character creation and advancement. Characters will select talents from class-specific lists as they gain levels. Armour and weapon proficiencies are just particular types of talents.
Unlike d20 Modern or D&D, Ground Zero characters don't gain additional talents just for gaining levels - they only receive them from their class(es). Similarly, characts don't get automatic attribute bonuses every 4 levels; instead, characters must select talents such as "Increased Strength", "Increased Constitution", etc.
There's lots of other details that I've "adjusted" over the years from canon d20. Please realize before you join the PBeM that I'm one of those "tinkerer" GMs who loves running homebrews. After all - one of the benefits of being GM for Ground Zero is that I get "Rule Zero" anything I want!
Some other homebrew rules that I can think of:
I'll add more items to this list as I think of them.