This is not intended to be comprehensive or authoritative, just free online resources I've found valuable while learning more about Erlang.
- 0xAX's list of Erlang bookmarks
- Federico Carrone, Erlang Spawned Shelter
- Ivan Uemlianin's list of resources on various BEAM languages
- David Robakowski's curated list of awesome Erlang libraries, resources and shiny things
- Julius Beckmann's curated list of amazingly awesome Elixir and Erlang libraries, resources and shiny things
- Erlang Master Classes (videos)
- Learn You Some Erlang
- Erlang in Anger
- Erlang Handbook: A concise reference for Erlang
- Concurrent Programming in Erlang / Part 1 (PDF)
- Try Erlang (hands-on web-based demo)
- Erlang for C, C++ and Java Programmers (very short overview)
- Joe Armstrong's Ph.D. thesis (PDF)
- Introduction to Erlang blog series
- The Architecture of Open Source Applications: Riak and Erlang/OTP
- Getting Started with Erlang User's Guide
- OTP Design Principles User's Guide
- BEAM VM Wisdoms
- Official Erlang docs
- Alternative presentation of Erlang docs
- This site is particularly useful if you need to check docs for older versions of Erlang/OTP
- Inside the Erlang VM with focus on SMP (PDF)
- Erlang: The Movie
- Erlang is not a Concurrent Functional Programming Language
- Systems that Run Forever Self-heal and Scale: Lambda Jam keynote by Joe Armstrong
- Erlang's Open Telecom Platform (OTP) Framework: QCon talk by Steve Vinoski
- Erlang – A survey of the language and its industrial applications (PostScript)
- Why Erlang Is Awesome
- When would you choose Erlang?
- Erlang's Big Idea
- Comparing C++ And Erlang For Motorola Telecoms Software
- Solving Embarrassingly Obvious Problems In Erlang
- More Embarrassingly Obvious Problems
- Programming Rules and Conventions (incomplete)
- GitHub Erlang repositories
- Comprehensive Erlang Archive Network (unclear if still active) – offline currently, related repository at https://github.com/cromain/cean
- Idris2-Erlang: Erlang code generator for Idris 2
- Caramel: An Erlang backend to the OCaml compiler
- Joe Armstrong's new dialect of Erlang
- Luerl - Lua for the BEAM
- Alpaca, the successor to MLFE
- ML Flavoured Erlang (MLFE) (now see Alpaca)
- Erlang flavored by Some ML
- Joxa
- Lisp-flavored Erlang (LFE)
- Elixir
- The Concurrent Schemer (CSCM)
- Efene
- Erlog, Robert Virding's implementation of Prolog in Erlang
- Erlyjs, JavaScript in Erlang
- purerl - the PureScript Erlang backend – Blog post
- Elchemy, Elm-like syntax for Elixir
- ePHP, PHP for the BEAM
- FEZ - an fsharp to core erlang experiment
- Reia
- Clojerl, Clojure for the Erlang VM
- Gleam
- Generate Erlang VM byte code from Haskell
- Hamler: Haskell-style functional programming language
- OTPCL: Open Telecom Platform Command Language
- Pony
- Orleans
- Akka.NET
- CAF - C++ Actor Framework
- SObjectizer
- QP
- Humus
- Proto Actor
- Cloud Haskell
- E
- Swish
- nact
- Bastion
- Actix
- Lunatic
- All you wanted to know about the HiPE compiler (but might have been afraid to ask) (PDF)
- Erlang - Programming the Parallel World (PDF)
- Joe Armstrong on GitHub and Blogspot
- Jesper L. Andersen on Medium and Blogspot
- Paolo D’Incau
- Takeru OHTA (Japanese)
- See also http://qiita.com/sile
- Ward Bekker
- Erlang Battleground by Brujo Benavides
- Federico Carrone
- It's Actors All The Way Down by Dale Schumacher
- History of Erlang
- One Pass Real-Time Generational Mark-Sweep Garbage Collection (1995)
- The development of Erlang (PDF)
- Implementing a functional language for highly parallel real time application (PostScript)
- Erlang - A survey of the language and its industrial applications (1996)
- An Introduction to Core Erlang (PostScript)
- Bjarne Däcker's thesis (2000)
- Comparison of Erlang Runtime System and Java Virtual Machine (PDF)
- Testing Erlang Concurrency with QuickCheck
- Note: QuickCheck is commercial software, but tremendously powerful and accessible as a web resource via QuickCheck CI
- Actor Model of Computation: Scalable Robust Information Systems / Hewitt, 2012 (PDF)
- The Actor Model (video)
- Actors and Continuous Functionals / Hewitt & Baker, 1977 (PDF)
- A Universal Modular ACTOR Formalism for Artificial Intelligence / Hewitt, Bishop, Steiger, 1973 (PDF)
- Actor-Oriented Models for Codesign (PDF)
- ACTORS: A Model of Concurrent Computation in Distributed Systems / Agha, 1985
- Message Passing and the Actor Model – chapter from Programming Models for Distributed Computation
- Relation to Pi Calculus from
erlang-questions - Modeling Erlang in the Pi–Calculus (PDF)
- π-calculus (Wikipedia)
Thanks @eao197, have added SObjectizer and QP, but not Just::Thread Pro since it doesn't fall under the rubric of "free" online resources.