For users

What are language tags?
Why tagging?
Tag wisely
The Wikipedia article
The excellent article from the W3C about language tags
Also from the W3C, a tutorial "Declaring Language in XHTML and HTML"
Other resources
Search subtags online in the registry (some explanations are provided, but this tool is more useful if you know the concepts behind the registry)
Browse language tags and see their meaning

For software developers

The subtag registry (version ) in various formats
Mark Davis' parser (ICU), written as a regexp suitable for Perl or Java
Dave Pawson's implementation in Java
Martin Dürst's implementation in Ruby is available as a Gem named "langtag" at RubyForge
Stéphane Bortzmeyer's implementation in Haskell.
Addison Phillips' code, as a regexp for Java
An implementation in XSLT (requires a XSLT 2.0 processor) and the Web page it powers (by Felix Sasaki).
Test suites

Tout est aléa, confusion et précarité, sauf le Catalogue. (Fred Vargas)