This is not intended to be comprehensive or authoritative, just free online resources I've found valuable while learning more about Erlang.
- Learn You Some Erlang
- 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
- Concurrent Programming in Erlang / Part 1 (PDF)
- 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)
- Solving Embarrassingly Obvious Problems In Erlang
- More Embarrassingly Obvious Problems
- Programming Rules and Conventions (incomplete)
- 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)
- Actor Model of Computation: Scalable Robust Information Systems / Hewitt, 2012 (PDF)
- The Actor Model (video)
- Actors and Continuous Functionals / Hewitt & Baker, 1977 (PDF)
- Why Do Computers Stop and What Can Be Done About It? / Gray, 1985 (PDF)
- Relation to Pi Calculus from
erlang-questions - Modeling Erlang in the Pi–Calculus (PDF)
- π-calculus (Wikipedia)
Twitter seems to be the de facto water cooler for technical conversations, and Erlang is no exception.
There are far more active accounts than I can comfortably list here; these are just a few of the more interesting ones.