What do you get when you take Dungeons & Dragons and put it in the 21st century? d20 Modern.
Back in the good old days of 3e (not 3.5), some brilliant bastard at Wizards decided that there was no point in having only one game using the d20 System. So, they took the D&D rules, replaced the classes, and added guns. The result was d20 Modern, an easy-(enough)-to-play modern RPG (if you don't mind rules skewed toward magic and monsters, as opposed to, you know, REALITY).
Like D&D, Wizards also belched forward a number of accessory books for d20 Modern, which, unlike the D&D supplements, were few in number and actually worth buying. They still handled guns with shitty attempts at realism, but then every RPG does.
If you look carefully, you'll see some of the ideas from d20 Modern survived into D&D 4e, which some would say is a brief glimpse of win midst the fail, or just more failfuel for the failtrain. Or if you're idealistic, hopelessly naïve, and/or retarded, a glimpse of win piling onto epic win.
Another nice thing that kept the guns SOMEWHAT realistic, was massive damage threshold. Instead of 50 like in D&D, it was equal to your Con score. This meant a critical hit or good roll from a gun (usually 2d6 to 2d10 damage) could force you to make a save or instantly go to -1 hit points. If gun damage had been buffed a bit, it could have been a nice alternative to the "Vitality and Wound points" used by Star Wars d20.
Differences from D&D
- Action points, like 'karma' or 'luck' points in other games. You can use them to either boost the number you rolled on d20, or to use a feat that requires an action point (usually to do something totally awesome). Action points spent do not return until you level up.
- You didn't keep track of how much gold/dollars/nuyen you had; instead, Wealth was a skill check, and getting big wads of cash was a bonus to your Wealth rolls. Purchases had a DC for Wealth checks, and buying something with a higher DC than your Wealth score (which was virtually anything not one use) meant you lost some bonus (because you spent beyond your means). It was horrifically, horrendously, broken. Not in the "overpowered" sense but "they didn't playtest this once".
- Having anything with a non-trivial cost reduced your wealth bonus by at least 1. A starting character's wealth bonus is (2d4+1 to 4 from starting occupation+1 from having ranks in profession. Typically 7). A Computer (-2d6 because computers cost an absurd amount in this system), a printer/scanner (-2 because these are completely different items), filling out a 4473 form (-1), a shotgun (-1), a decent backpack (-1), and a single set of clothing (-1) will take a character from "middle-class" to impoverished.
- It also has some really fucking bizarre ideas on how restricted everything is legally. Suppressors ($200 tax and a bunch of waiting) are, under the game's rules, harder to get than full auto (The same, but banned since 1986 so all grandfathered examples cost tens of thousands) while sawed off shotguns (Same as suppressors) are flat out impossible to get legally in the game's rules (and black market sawed off shotguns are several times more expensive than normal black market shotguns).
- No alignment system; instead you had an "allegiance." You could have an allegiance to a religion, a nation (patriotism), an organization, a philosophy... and you'd get a +2 circumstance bonus when dealing with people that have the same allegiance.
- In fact, you could have multiple allegiances, although most people were too dumb to take advantage of that fact.
- You had an Occupation, which was like your level 0 class that gave you some class-skills and maybe a bonus feat.
- There were three noticeable tiers for characters: levels 1-7, levels 8-15, and levels 16-20.
- Every class only has 10 levels; you had to take an "advanced" (read: prestige) class or multiclass if you wanted to go higher.
- There are six basic hero classes, one for each of the six base attributes: Strong, Fast and Tough heroes (Str,Dex,Con), and Smart, Dedicated and Charismatic heroes (Int, Wis, Cha). There's no penalty for multiclassing (indeed, it is required by level 11, see above). There were 12 advanced classes, designed as 2 for each basic class, but which could be taken by any character provided appropriate requirements were met by said character. Some splats added additional advanced classes, as well as some 5-level true "prestige" classes.
- Basic classes had no class features but got talents from a class specific list every odd level. Most were just plain old shit and the worst class features ever (You get +1-3 to a couple skills! If you take longer you can lift/bend harder!) but the system would eventually be fixed and made awesome in Star Wars: The Roleplaying Game Saga Edition before being made useful but shit in a completely different way in 4th Edition Dungeons and Dragons.
- No spells/miracles/psi (called "FX abilities") for base classes. Only prestige classes can have them, and refreshingly little of the rulebook is dedicated to listing and describing spells.
- Spellcasting classes are all advanced classes, so you can't enter till level 4, and like all classes only go up to level 10 limiting you to 5th level spells. Theoretically this makes them less powerful, but practically it just means any mage is a Gish to some degree.
- Had several supplements, including d20 Future and d20 Past. d20 Future had rules for playing (you guessed it) in the future and also had several supplements (d20 Apocalypse, d20 Cyberscape and Future Tech). d20 Past, meanwhile, had rules for playing the past. Obviously. The supplements allowed players to engage in updated-for-a-new-generation games that Gary Gygax had put out, including Gamma World, Boot Hill, Star Frontiers, Age of Exploration, a Zombie-Apocalypse variant, and many, many others. Suffice to say, these supplements, unlike their counterparts in Dungeons and Dragons, were actually worth buying and checking out.
- Child character are explicitly playable, but it's impossible to complete character creation with one unless their dexterity was high or their strength really high. Fixed in splat.
- While magic items aren't excepted, mastercraft ones are trivial to get for someone who can craft, but GM's whims for one who can't, and give the same bonuses. This reinforces the only crafting skill worth a damn being mechanical, as that covers weapons, armors and vehicles.
- d20 Modern System Ref Document hypertext open-gaming license book of the whole enchilada.