Apps:
PoE Addon Launcher (Basically launcher for other apps): https://github.com/POE-Addon-Launcher/PAL2

Building:

Path of Building (Offline build planner for Path of Exile.): https://github.com/Openarl/PathOfBuilding

PoESkillTree (A Passive Skill Tree and Character Planner for Path of Exile): https://github.com/PoESkillTree/PoESkillTree

Currency & Trading:

PoE Trade Macro (Check item prices in game): https://github.com/PoE-TradeMacro/POE-TradeMacro

Mercury Trade (Easy trading, preset replies and popup of item offers): https://github.com/Exslims/MercuryTrade

PoE Trades Companion (Easy trading, preset replies and popup of item offers): https://github.com/lemasato/POE-Trades-Companion

XenonTrade (Price checking/trading overlay): https://github.com/klayveR/xenontrade

Trader for PoE (Yet another trading app): https://github.com/labo89/TraderForPoe

Acquisition (Trade items without premium tabs, find items in your stash): https://github.com/xyzz/acquisition/

Currency Cop (Calculates value of your tabs in Chaos Orbs): https://github.com/currency-cop/currency-cop

Exilence (Track net worth, maps, gear and more for you and your party): https://github.com/viktorgullmark/exilence

Exile Diary (Track drops, Exp and profit in your maps and more!): https://github.com/briansd9/exile-diary

PoE Dashboard (Create dashboards with currency ratios and item prices): https://github.com/5k-mirrors/poe-dashboard

TradeMaster (Buy currency and maps without leaving the game!): https://github.com/StefanKunde/TradeMaster_POE

Leveling:

PoE Level Buddy (Displays useful info while leveling): https://github.com/Pastillage/PoE_Level_Buddy

Poe Leveling Overlay (Shows possible layouts of an area to make leveling faster) https://github.com/Biggoron144/PoE-Leveling-Overlay

Path of Leveling (Plan Leveling, Navigate thought zones and see if you are over/under-leveled): https://github.com/karakasis/Path-of-Leveling

PoE Direction Helper (Leveling overlay with zone layouts): https://github.com/flacostec/PoEDirectionHelper

Lab Compass (Labyrinth Map based on current layout): https://www.poelab.com/lab-compass/

PoE Crafting Bench (Crafting Recipes, Prices and Locations): https://pvaass.github.io/poecraft/

Misc:

Filtration (Create your own Item Filter the easy way using intuitive GUI): https://github.com/ben-wallis/Filtration

PoEHelper (Yet another overlay. Check Fossils, Maps, Syndicate members etc.): https://gitlab.com/vtopan/poehelper

Filter Bro (Update many popular ItemFilters to it's newer versions): https://github.com/Skyline969/FilterBro

PoE Controller (Play Path of Exile using a controller [Not as good as Xbox One version... yet]): https://www.pathofexile.com/forum/view-thread/1594688

YoloMouse (Change your cursor appearance to make it more visible): https://pandateemo.github.io/YoloMouse/

LutBot (PoE AutoHotkey Macro with many options): http://lutbot.com/#/

PoE Racing (Racing companion and overlay): poe-racing.com

Websites:
Grinding.Zone (Hub with most of these links): https://grinding.zone/

Building:

Poe Builds (Easy to navigate list of Path of Exile builds): https://www.poebuilds.cc/

PoE Build Browser (Another Easy to navigate list of Path of Exile builds): http://timtips.com/buildbrowser/

Mikelat's Aura Calc (Online Aura reservation planner): https://poe.mikelat.com/

PoE Planner (Plan your skill tree online): https://poeplanner.com/

Currency & Trading:

PoE Trade (Most popular Path of Exile shop indexer): http://poe.trade/

Official PoE Trading Site (Official trade site made by GGG): https://www.pathofexile.com/trade

PoE App (Yet another trading site with some new options): https://poeapp.com/

Poe Ninja (Currency/Item prices and past graph info): https://poe.ninja/

PoE Antiquary (Check archival prices of items from the past leagues): https://poe-antiquary.xyz/

PoE True Bulk Map Trading Site (Trade Site for buying maps in bulk): https://easypoe.herokuapp.com/

Vorici Chromatic Calculator (Check probability of using chromatic/recipe on item): https://siveran.github.io/calc.html

Leveling:

PoE Lab (Current Lab layouts, updated daily): https://www.poelab.com/

ExileNinja (Plan your leveling progress before league): https://exile.ninja/

PoE Road-map (Use Path of Building code to create leveling road-map): http://poe-roadmap.com/

Misc:

PoEDB (Data-mined info about items/gems/skills etc.): http://poedb.tw/us/

PoEAffix (Path of Exile item affixes): http://poeaffix.net/

Path of Exile Wiki (Well, it's Wiki, you should know what that is): https://pathofexile.gamepedia.com/Path_of_Exile_Wiki

PoE Checklist (Checklist of all the maps (Even these that are not on the Atlas): http://poechecklist.com/

FilterBlade (Customize NeverSink filter to your own liking): https://www.filterblade.xyz/

FilterBlast (Customize many popular filters to your own liking): https://filterblast.xyz/

TLDR Exile (PoE forum in more responsive version): https://tldrexile.com/pathofexile.com/forum/