The Open XML Developer Wiki provides a variety of content that introduces Open XML and helps you get started.

About Open XML
Benefits of Open XML
Open XML Developer Scenarios
Comparison of Open XML to ODF
About Open Packaging Conventions
About WordprocessingML
About SpreadsheetML
About PresentationML

Open XML Tools and Specialized Programming Interfaces

Resource Center: PowerTools for Open XML
Resource Center: DocumentBuilder
Resource Center: PresentationBuilder
Resource Center: Open XML and JavaScript
Resource Center: Open XML SDK for JavaScript
Resource Center: Open XML SDK 

Screen-cast Series

Introduction to Open XML
Recursive Pure Functional Transforms of XML

Open XML Content List

Open XML content by keyword
Open XML content by author