Continuing my tradition of starting documentation and references threads, I here start an XML references thread with the intention of making it as complete and useful as my JavaScript or HTML&CSS threads. Let's start with the material from W3C:
First, a quick guide to the W3C technical reports levels:
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Rec: Recommendation, this is the final, definite specification. For public use.
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PR: Proposed Recommendation, this is the last instance at which changes may be made to the specification itself, and should see very few changes when it hits Recommendation status. For public use.
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CR: Candidate Recommendation, this is the stage at which W3C calls for implementations. For implementors.
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LC: Working Draft in Last Call, this is a stage where the specification is relatively complete, but it may still take major changes. Not for use.
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WD: Working Draft, the stage where W3C asks for public review, comments and input. A specification may stay a Working Draft for a long time and go through many radical changes during that time. Not for use.
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Note: Not a W3C promoted technical report, but it may be a clarification from W3C, a suggestion from the W3C or a W3C member, a proposed specification from a W3C member etc.
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Errata: Corrections to and clarifications on specifications that has already reached Recommendation status.
XML Core Specifications:
XML Extensions:
XML Validation:
XML Applications:
Tutorials, References, Examples & Implementations:
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CS: Committee Specification
IS: International Standard
Before you post, read our: 
Well, now almost all XML related W3C technical reports that are not XML applications in their own are in the list. XSL and related technical reports too, together with non-W3C Relax NG.
