Credits go to NotEvenDoomMusic, Ciunek, and PyaKura for showing me these; I'm also using their quotes.

Kiranico (MH3U site | MH4U site)
- One of the best sites to gather resources for your quests, monster data, items and basically everything you need to know.
- It's like a database for everything existing in the MH3U and MH4U universe.

MHPedia (MH3U)
- Just like Kiranico it's a database on things like Items, Armor, Droprates etc. but overly simplified and is displayed minimalisticly.
- It's also very easy to pick and find what you need without other gimmicks.
- Great for beginners.

Monster Hunter Armor Generator
- It's an extensive page with details about all existing armors with lots of other additions.
- The page is also very useful for previous games of the MH3 series like the regular Tri, Portable 3rd, 3G and of course 3. It recently also supports MH4

MHAG for Windows (external program)
- Offline database just like Kiranico and MHPedia that also comes with the bonus of an visual aid in terms of where monsters are vulnerable and which elements are strong on which bodypart.
- In addition of being an offline database, it literally covers almost EVERYTHING in terms of what you need to know.
- In my opinion more user friendly for experienced players.
- Can also be merged with Athena's Armor Set Search (can be found seperately but the program has an built in option to download it) that is capable of searching out the best armor & decorations if you specifically need a certain skill combination for your own purposes.
AASS will generate the armor set that you need, for an example if you need Quick Sheathe and Speed Sharpener, it will generate a "possible" armor solution.

(MH3U only) There's also something called Athena's Charm Table, which is also an external program that lets you figure out which Charm Table your character is using.
It's very advanced and would only be for importance, for the people who take their MH savegame very serious.
In laymans terms what it does and what a Charm Table is: (i'll try to explain it in my own words)
Every game you start has it's own Charm Table that you start with that is ranging up from 1 to 17, it's "quite" RNG for casual players, but for experienced players can be manipulated by doing various things.
It's for importance to those that are very serious about getting the best possible equipment for their class, because a Charm Table represent the set amount and type of "Charms" you can receive.
Your character will be locked with that table of charms for the entire savegame until you start a new character.

(A detailed explanation can be found here.)

Concerning MH4U's Charm Tables:

Charm tables are now reset everytime you load up the game from main menu, you're not locked at chara creation like in 3U (thank god).

People on 4chan's /mhg/ were posting few sets and methods for charm farming:

1) Charm God, Gathering+1
>2 Slot weapon (2 Gathering gems)
>Torso up
>3 slot torso (3 Charmer gems)
>Derring Vambraces S (1 Gathering gem)
>Torso Up
>Torso Up
>+8 Charmer talisman

2)Set:
3 slot weapon
Skull Visage (or anything with Torso Up)
Chakra Necklace (or anything with 3 slots)
Leather Vambraces S
Lagiacrus Faulds X (or anything with 3 slots)
Tetsucabra Greaves X (or anything with Torso Up)
Charmer +7 2-slot charm

Gathering Jewel x6
Charmer Jewel x5 (enjoy farming the Rath Gleams by the way)

Gathering God
Charm Chaser

Charm Chaser being so hard to get due to needing charm and farming those silver/gold faggots, you can instead go for Iron Belt and Speed Gathering if you want.

Now for the quest:
>Eat Drink + Fish for Felyne Explorer
>Pick 10* Caravan Quest titled Puffer Fish
>Start quest, mine and gather everything in the secret area
>Jump down and climb to Area 3
>Kill the 3 Zamites
>Go up the hill and mine whatever's there
>By the time you're done mining them, 2 more Zamites spawned, kill them
>End via subquest
>Repeat

Video tutorials and showcases (MH3U and MH4U)
- It's a great series of videos to get into MH3U and has some really helpful tricks to obtain for example, certain items that are harder to gather than others.
- He does tons of general hunting and does like for an example, MH4 weapon showcases aswell etc.

Weapons: pros and cons (MH3U)

GaijinHunter's Tumblr
- Many helpful advices and tips there, along with the possibility of him answering any question about MH you throw at him.

E3 2014 footage
Trailer from July 11, 2014's Nintendo Direct (JP)
Tokyo Game Show 2014 trailer
Weapon design contest winner (EU)
Weapon design contest winner (Americas)
Palico walkthrough
Super Mario Bros. & The Legend of Zelda crossovers (SMB stuff out already; Link stuff to come March 6, 2015)
Sonic the Hedgehog crossover
Metroid crossover
Megaman crossover
Street Fighter II crossover
Devil May Cry crossover
Animal Crossing crossover
Taiko crossover
Article about Guild Quests being changed
Article about MH4U's localisation and more

There's also a MH4U 13-part parody mini-series on the official Monster Hunter YouTube channel. You can watch the first episode here.