Open Source Erlang/OTP can be downloaded from the open source Erlang site. The site provides full source code as well as binaries for Windows machines. This is probably the best place to go to make sure you're getting the latest version.
The Debian Distribution of GNU/Linux, and derivatives such as Ubuntu, includes various versions Erlang in different releases, under interpreters.
Precompiled ready-to-go installers for a variety of platforms are available from the Comprehensive Erlang Archive Network (CEAN). CEAN also provides dozens of Erlang applications.
For the BSDs, the 'ports' collection has up to date versions of Erlang.
Users within Ericsson can use Ericsson's internally supported version of Erlang from www.erlang.se.
Erlang/OTP 17.0 was released in April 2014.
Prior to 17.0, Erlang used a slightly unusual version numbering scheme, e.g. the previous version was called R16B-3.
Major releases typically come somewhere between 12 and 18 months apart.
Components of Erlang have their own version numbers, for instance the virtual machine in 17.0 has version 6.0.
The Open Source Erlang Licence is essentially the Mozilla (Netscape) Public Licence with a few modifications to make it compatible with Swedish law.
As far as I understand, this means you can obtain Erlang for free, use it to build cool systems and sell them without Ericsson coming around to charge you money. For an authoritative statement, you'll need a lawyer.
The language itself is best described by the Language Reference Manual.
The documentation delivered with open source erlang, as both man pages and html, is the most up-to-date reference to both the OTP libraries and Erlang itself. On unix systems, an easy way to read the man pages is erl -man gen_tcp .
Different books are aimed at different audiences, so recommending one in particular is difficult. The currently available books in English, starting with the most recently published, are:
Introducing Erlang by Simon St. Laurent is a very gentle introduction to Erlang for beginners. It's available directly from the publisher.
Erlang and OTP in Action by Martin Logan, Eric Merritt and Richard Carlsson was published in 2010. You can buy it from various online bookstores, or directly from the publisher.
Erlang Programming (A Concurrent Approach to Software Development) by Francesco Cesarini and Simon Thompson was published in 2009. Get it from Amazon etc or directly from the publisher.
Programming Erlang: Software for a Concurrent World (Joe Armstrong) is a book about Erlang written by one of the original inventors, published in 2007. You can buy it on paper from Amazon and other online bookstores, or directly from the publisher.
There are also some books in languages other than English:
German: Erlang/OTP (Plattform für massiv-parallele und fehlertolerante System) (Pavlo Baron) can be ordered directly from the publisher or via Amazon.
Spanish: Erlang/OTP Un Mundo Concurrente (Manuel Rubio) can be ordered directly from the publisher.
Of historical interest only: Concurrent Programming in Erlang (2nd. Edition, 1993) (Armstrong, Virding, Wikström & Williams). Archaeologists can download the first half of the book from erlang.org.
Also only of historical interest: Maurice Castro's book, Erlang in Real Time (1998).
The Standard Erlang spec aims to define the Erlang language. There is currently no compiler which actually implements everything exactly the way Standard Erlang specifies. This specification is quite precise; it's intended as a reference, not a bedtime read! Compiler and tool implementors will find this useful, as will advanced Erlang programmers.
Core Erlang is an effort to define an Erlang-like language which is nicer to compilers and language tools and can be automatically generated from Erlang. Erlang compilers from R10 onwards can generate Core Erlang from normal Erlang code. They can also generate .beam files from Core Erlang.
The Erlang Rationale is a short (13 page) discussion written by one of the original authors of Erlang about why some parts of Erlang became the way they are. It's a work in progress.
The proceedings from the Erlang User Conferences contain many interesting articles. The proceedings for each of the yearly conferences since 1999 are online.
The internals of the BEAM file format are described on Björn's homepage . Eventually this will/might include a description of the virtual machine's instructions. Björn also includes some benchmarks comparing different versions of the BEAM machine. You may also want to take a look at the beam_lib module.
Joe Armstrong's Thesis, Making reliable distributed systems in the presence of software errors contains a lot of interesting information, including a compact overview of the language, a number of case studies and some discussion of the language's evolution, somewhat similar to Bjarne Stroustrup's Design and Evolution of C++.
This Masters Thesis contains a good overview of some of Erlang's implementation aspects, including memory allocation (section 7).
Another Masters Thesis contains a good overview of Erlang's distribution mechanisms (Section 5).
The (Ex-Ericsson) computer science laboratory has a collection of papers about Erlang-related topics.
The source-code release includes a directory with some of the internal build documentation in the subdirectory erts/emulator/internal_doc/ .
There are a couple of books about specific types of applications which use Erlang as the main language in the book:
Building Web Applications with Erlang by Zachary Kessin
Handbook of Neuroevolution Through Erlang by Gene I. Sher
The standard text about data structures in functional languages is Purely Functional Data Structures by Chris Okasaki. The parts about laziness (about half of the book) do not apply to Erlang, but are interesting nonetheless. An early version of the book is available online, the book itself is available from many online booksellers.
Structure and Interpretation of Computer Programs is a standard introductory text to functional programming. It is available both on paper and online.
When you write Erlang code you want to share, there are quite a number of ways to share it. The place to announce such things is the Erlang Questions mailing list, or, in the case of patches, the Erlang Patches mailing list. Where you put the actual code is up to you.
If your code is a correction, modification or extension to an existing part of OTP, the usual way of spreading it is via github, the wiki has instructions.
If your code is a new application or library, some common ways of distributing it are:
Putting the code on a personal homepage
Getting your code packaged into the CEAN
Even though Erlang is a fairly mature language with a large established code-base, changes and enhancements are still being incorporated. There is a mechanism called the Erlang Enhancement Proposal which formalises the process.
Erlang Factory organises regular Erlang conferences around the world. By tradition, the annual Erlang User Conference is held in May in Stockholm (Erlang's birthplace).
Academic conferences about functional programming are also attended by a few Erlang users. The International Conference on Functional Programming usually includes an Erlang workshop.
Every so often, someone puts out an invitation on erlang-questions to informally discuss Erlang over a few beers. Past Erlounges have been in Sweden (Stockholm), Scotland, the USA (Atlanta, New York, Chicago, North Carolina, Schaumburg, Seattle, SF Bay area, Arlington, Washington DC), France (Paris), China (I can't read the characters which described the location), Italy, Germany (Aachen), Poland (Krakow), New Zealand (Wellington) and others. There's a Trapexit page listing known usergroups.
There are several mailing lists devoted to Erlang:
This list carries announcements about new releases of Erlang, patches, etc. All posts to erlang-announce also appear in erlang-questions.
This list is for patches to the Erlang distribution. At the time of writing, it is not archived.
This list is for bug reports.
The erlang-questions list is archived on the Erlang website.
The trapexit site also has a forum interface to the mailing list.
There's an IRC channel, #erlang, on irc.freenode.net
Posting a short note to email@example.com is a good start, especially if the subject-line clearly identifies the post as a job request/advert.
There are several sites with a high concentration of Erlang jobs:
If you want to be listed here, send some mail to firstname.lastname@example.org.
Kontakten (Contact), an Ericsson-internal newspaper, may also be useful as it carries many job ads.