Usenet

Usenet - Usenet is a worldwide distributed discussion system available on computers.
Usenet newsgroup - A Usenet newsgroup is a repository usually within the Usenet system, for messages posted from many users in different locations using the Internet.
r/Usenet - A thriving community dedicated to helping users old and new understand and use Usenet.
Getting started with usenet - /r/usenet wiki
r/UsenetInvites - Requests and offers for Usenet indexers.
NZBLINK - The NZBLNK™ URI scheme defines the format of NZBLNK™ links to identify binary Usenet content and supplies some extra information to handle that content correct (similar to magnet links, but for NZBs).
Usenet-Uploaders - Table of applications for uploading content to Usenet.
QuickPar - Tool for reconstructing damaged/missing/corrupt Usenet binaries.
NZB Monkey - NZB download helper-tool.
SABconnect++ - Chrome extension which adds one-click 'Send to SABnzbd' buttons to many popular NZB index sites.
ngPost - Command Line (or minimalist GUI) - usenet poster for binaries developped in C++/QT.
A Quick Guide to Choosing a Usenet Provider - Reddit post by /u/FlickFreak.

Forums

Revenge of Usenet - Forum for usenet related news, and download discussions.
Brothers of Usenet - The original board went down, so this is a mirror for the well known forum which discusses stuff around the scene.
Fileleechers - A young forum around Usenet.
House of Usenet - New provider, new url, a reboot of the well known forum.
omgwtfnzbs - (paid) - A paid usenet indexer.
SceneNzb.com - (paid) - 3 NZD files per day are free.
Sky of Usenet - Yet another german usenet forum.

Usenet Express - UsenetExpress is a powerful new tier-1 Usenet provider that offers strong security, a 10GB uplink per server and up to 150 streams for an excellent price.
Eweka News - Netherlands-based Usenet provider.
Newsdemon - Cheap and cheerful Usenet provider with frequent discounts.
Newsgroup Ninja - \$7.99 per month, SSL encryption, Unmetered usage, Unlimited speeds.
Premiumizer - A download management tool for premiumize.me cloud downloads.
Tweaknews - Dutch Usenet provider that offers a Highwinds - news feed.
Usenet Crawler - Movies, eBooks, TV Series, Anime & more.
Usenet Providers and Backbones - This is a simple overview of the current companies, backbones, providers and resellers in the Usenet landscape.
Usenet.Farm - Usenet reseller with 1000+ days retention.
WorldSrc - Movies, software, apps, games, music, and images available for fast direct download + torrents.
XS News - European Usenet feed.

r/Usenet wiki: indexers - Information about /r/Usenet's favourite indexing services.

DOGnzb - Invite-only NZB site (although they do have a registration page that occasionally opens up while maintenance).
DrunkenSlug - Popular NZB indexer with a free tier and decent retention.
NZBCat - NZBCat is an invite-only nZEDb NZB indexer.
NZBFinder - Usenet indexer and newznab API with a clean UI and 8+ year backlog of NZBs.
NZBgeek - Affordable Usenet indexer operating since 2014.
omgwtfnzbs - Invite-only NZB indexer with a funny name.

BINZB - Minimalistic indexer.
Binsearch - With this site you can search and browse binary Usenet newsgroups.
FindNZB - One old indixer, the results are outdated and often offline.
NZBIndex - The first free Usenet indexer you find in your Google search results.
NZBFriends - One of the oldest indexers, sadly the results are often outdated too.
NZBKing - The service allows you to search and browse binary files that have been posted to Usenet newsgroups.
Usenet Crawler - Usenet indexer with API access for registered users.
NewzLeech - Newzleech is a Usenet file search engine.
GingaDADDY - Another popular free NZB indexer, requires sign-up.

Momentum Plus - Usenet-Newsreader and in the (near future) - indexer and collector for usenet files.
nZEDb-deploy - A collection of scripts to automate and simplify the deployment of a nZEDb Usenet Indexer using the new format of their GitHub repository.
nZEDb - A fork of nnplus(2011) - | NNTP / Usenet / Newsgroup indexer.
newznab-tmux - Laravel based usenet indexer.
newznab - Newznab is a usenet indexing application, that makes building a usenet community easy.
NzBlite - (nzblite client) - Open source file format to simplify Usenet uploading and downloading to newsgroups.

NZBget - Efficient Usenet downloader written in C++.
NZBVortex 3 - Simply the best Usenet client for Mac.
alt.binz - A powerful binary newsreader, for downloading and managing articles from Usenet.
Newsbin - Newsbin is software for Microsoft Windows Operating Systems that downloads files from Usenet Newsgroups.
NZBGet - Efficient Usenet downloader written in C++ which is bit lighter on system resources than SABNZBd.
PowerNZB - The Ultimate Binary Usenet Client for Android.
SABnzbd - SABnzbd is an open-source binary Newsreader written in Python.
spotweb - Spotweb is a decentralized Usenet community based on the Spotnet protocol.
Unison - macOS app for accessing Usenet Newsgroups and the many wonders and mysteries contained within (discontinued).
Usenetic - The full-featured Usenet client for macOS.