Maybe not everyone knows that Sitecore has released a great deal of documentation for Sitecore 6 on the Sitecore Developer Network (SDN - http://sdn.sitecore.net). In general, the references describe concepts, while the cookbooks describe implementation. For developers, a good place to start is the Sitecore 6 Resources page. I think the best new resources include:

• The Glossary of terms and definitions.
• The What's New in Sitecore 6 document.
• The Installation, Installation Troubleshooting, Internet Explorer Configuration, Staging, Active Directory (AD), Team Development, Configuring Production Environments guides, and the Security Hardening Cookbook.
• The Sitecore Intranet Portal (SIP) Installation, Active Directory (AD) Integration, Core Concepts, Main Settings, Skin Settings, Content Features, Cookbook, and User guides.
• The Security Reference, the Security Administrator's Cookbook, the Security API Cookbook, the ASP.NET Membership Providers implementation guide, the guide to Low-Level Sitecore CMS Security and Custom Providers, and the (Microsoft) CRM Security Provider.
• The Client Configuration Cookbook, describing configuration of Sitecore user interfaces including the Content Editor and the Page Editor.
• The Content Reference, the Content Author's Cookbook, the Content API Cookbook, and the guide to Reusing and Sharing Data, all describing data management.
• The Data Definition Reference, the Data Definition Cookbook, and the Data Definition API Cookbook, describing data structures.
• The Presentation Component Reference, the Presentation Component Cookbook, the Presentation Component XSL Reference, the Presentation Component API Cookbook, and the Presentation Component Troubleshooting Guide.
• The Sitecore 6.1 Rules Engine Cookbook.
• The guide to working with Dynamic Links.
• The Workflow Reference.
• The Online Marketing Suite (OMS) Analytics Configuration Reference and the Online Marketing Suite Cookbook.
• The Media Facilities resource.
• The guide to handling the HTTP 404 Page Not Found condition with Sitecore.
• The Package Designer pages.
• The Lucene Search Engine resource, for implementing the Lucene search engine with Sitecore.
• The WebDAV Configuration Guide.
• These miscellaneous resources about events and Sitecore query.
• The Cache Configuration Reference.

You can download the entire Sitecore 6 API as a Windows Help file. And you can download the Documentation Package containing most of these files. Additionally, I am trying to remember to monitor the Release Notes. You can access information about the various optional Sitecore modules from the SDN Products Page.

So far, the feedback on these documents has been quite positive. Use the email link in the footer on SDN to email the Sitecore documentation team with questions, corrections, and suggestions.

I will also maintain a list of links to blog posts, forum threads, and other resources that address common issues:

• Control item names and paths (also discusses command templates) [2].
• Lucene search index corruption.
• Reflecting on Sitecore.
• Publish at a specific time [2].
• Sitecore configuration factory.
• Sitecore wildcards.
• Delete old versions.
• Delete version in workflow.
• Redirect old URLs [2].
• Repeating groups of data template fields.
• Contributing to hared Source.
• Forwarding legacy URLs.
• Languages in URLs.
• Stress testing [2, 3, 4].
• Access preproduction content.
• Control access to UI features.
• The publishItem pipeline (and pipelines in general).
• Transform the response stream (ASP.NET filters).
• Import Media.
• Virtualization.
• Report items in non-final workflow states.
• Exceptions in sublayouts.
• Sitecore interprets item name in URL as language name
• Generate JavaScript dynamically.
• ~/link.aspx.
• Validate tags in Rich Text fields (HtmlAgilityPack).
• Extending image processing.
• Icon for PDFs.

Specific to XSL:

• Determine depth in XML document.
• Counting items.
• Comparing dates.
• Accessing media items.
• Include external link in navigation.

Not really documentation, but some Sitecore Shared Source projects I access frequently:

• Library (API enhancements and specific features)
• PublishingSpider (publish items referenced by an item in workflow)
• SublayoutParameterHelper (access parameters and data source passed to a sublayout).
• DefaultFieldEditor (page editor edit frame menu command to open the field editor showing all fields that don’t support inline editing and are not defined in the standard template).
• FieldValueComparer - Sort items (based on this post).
• LinkProvider - SEO URL features (based on this post).
• MediaUrlTransformer - Prefix media URLs in RTE fields with a leading slash character, optionally convert media URLs in RTE fields to lowercase.
• PageNotFound - Pipeline processors to handle the HTTP 404 Page Not Found condition.
• XslHelper - XSL extension method library.
• ItemNamingRules - rules engine conditions and actions to control item names.