2. mega mods
Even though Fallout 3 is remarkably complex by today's standards, once you get the gist of it it becomes ridiculously easy. To add insult to injury, Ðog only knows what got into Bethesda when they decided to implement a level cap at 20 (30 if you have the Broken Steel DLC), a limit you're most likely going to hit before you uncovered half the locations on the map. On the one hand, the game gives you a gigantic world full of exploration possibilities, and on the other hand it would seem to punish you for exploring too much. Playing a RPG is not quite the same thing when you can't level up anymore. In this section we have mods that make the game much more balanced, complex and challenging; to the point that, after playing a bit with them, it's impossible to conceive Fallout 3 without them.
Arwen Reduced XPR, by Arwen_Eve
It might look too small to be called "mega", but a very important mod nonetheless: Arwen Reduced XPR, as you might've guessed, reduces the amount of XP gained by killing enemies, lockpicking, hacking, et cetera; so you'll level up much slower, and thus hitting the dreaded level cap will take much longer.
FO3 Wanderers Edition - FWE, by Peekaboom
Hands down, the father of all mega-mods. If for some reason you can only use one mod, just let it be this one.
Aside from dozens of tweaks to improve combat, it adds a number of features such as: Alternative beginnings with several pre-designed characters (so you can skip the whole Vault 101 part, starting as an adult in the wastes); AP-dependant bullet-time; the ability to make short sprints (also AP-dependant); thirst, hunger and sleep mechanics (including a portable water purifier, mini-grill and a sleeping bag); a motorbike that you need to maintain and refuel periodically in order to fast-travel (no more automagic fast-travel at any moment, makes the game a lot like Fallout 2 with the car); new drugs with very interesting effects (some can even be used as weapons, jabbing the needle into enemies); a robust system for treating crippled limbs including a couple of perks; and then some.
Most of the changes are optional, all of them are highly recommended to get a much more complex and interesting game. Requires CRAFT y CALIBR.
Wearable Backpack - BlackWolf Backpack, by bunsaki
A backpack (several, actually, each one with a different carrying capacity) that you can buy from any trusty trader and allows you to carry more weight. It's very useful when using FWE, which, among other things, even adds weight to ammunition =O
Gore Tweaks, by 100rads
Not exactly a megamod, but more of a complement: It allows for the customization of gibbing probability. I use it because FWE pretty much eliminates the gibs, but to me they're a fundamental part of the game's spirit. Yay for exploding heads! =D
Marts Mutant Mod - MMM, by Mmmmmmm
The second best megamod of all times: Adds some new creatures and modifies the behavior and appearance of existing ones. Some of the changes are truly remarkable, such as making each creature in a group look slightly different to each other (so that -for instance- a pack of dogs looks like an actual pack of dogs, instead of an army of dog-clones), or having feral ghouls waking up over and over after “death” (even after they lose some limbs) until you blow their head off.
Weapon Mod Kits - WMK, by antistar
Workbenches can be used -with the help of the new “Mod Kits”- to modify weapons; adding auto-fire, optical scope, clip extension, and such.
Fallout Interoperability Program - FOIP, by Peekaboom
A handful of sub-mods that make the most popular megamods work together in sweet harmony.
Community Ammunition Library - CALIBR, by Tubal
Honestly, I don't remember what exactly this one is for, but it's just one single, tiny file. And it is required if you're using FWE, so there's that.
Community Resource to Allow Fanmade Tinkering - CRAFT, by Highsight
Adds a new sub-menu to Workbenches, so you can build new items such as repair kits, fuel for the bike, toolkits, et cetera. Required if you're using FWE.
Acknowledgements & appreciation: To Bethesda, of course, perplexing amount of screwups notwithstanding, noone can deny they made an incredibly kick ass game; to all those sick freaks that sacrificed their social lives so we could enjoy the programming marvels featured on this guide; and to KamioKachu, my lab molerat during the creation of the piece.
And remember: Don't feed the Yao Guai!
That is all.