A bunch of you guys have been waiting for this one: Extensibility and development resources!

• Microsoft Dynamics CRM 4.0 Software Development Kit (SDK) (SDK – Microsoft)
  This package contains the complete software development kit for Microsoft Dynamics CRM 4.0.
• Microsoft Dynamics CRM 4.0 Deployment Software Development Kit (SDK) (SDK – Microsoft)
  SDK for customizing deployments for Microsoft Dynamics CRM 4.0.
• Developing ISV Applications using Microsoft Dynamics CRM 4.0 (Document – Microsoft)
  How to create business applications for Microsoft Dynamics CRM 4.0.
• Application Development in Microsoft Dynamics CRM 4.0 (Article – MSDN)
  The following topics are covered in this article:

  · Creating an application that connects to Microsoft Dynamics CRM
  · ASP.NET applications on the Microsoft Dynamics CRM server
  · ASP.NET applications for Microsoft Dynamics CRM for Microsoft Office Outlook with Offline Access
  · Plug-ins
  · Custom workflow activities
  · Scripting
  · Setup and deployment
  · Application configuration

• Microsoft Dynamics CRM 4.0 Logical Database Diagrams (Visio Diagrams – Microsoft)
  A series of diagrams showing the logical database structure for Microsoft CRM 4.0.
• Microsoft Dynamics CRM 4.0 Developer Center (Site – MSDN)
• Microsoft Dynamics CRM Code Samples on MSDN Code Gallery (Site / Sample Code – MSDN)
• Stunnware’s JavaScript snippets directory (Article – Stunnware)
  The "More JavaScript" series now has 90 samples and is divided into five separate articles. Use this directory to easily locate the script that most interests you.
• Ascentium's Microsoft Dynamics CRM JavaScript SDK (JavaScript SDK – Ascentium)
  The CrmService.js script supports sending messages directly to the CRM 4.0 web service utilizing only JavaScript. You can use this file in your own ASPX pages, from with CRM forms or virtually anywhere.
  For many of the functions you can also pass in a callback function so they can execute asynchronously.
• Developer Ramp up Kit for Microsoft Dynamics CRM 4.0 (Videos, Slides, Labs – MSDN)
  This kit is for .NET based developers who want to ramp up and build on top of the Microsoft Dynamics CRM platform. Microsoft Dynamics CRM 4.0 uses familiar components such as Web Service endpoints, Windows Workflow Foundation, SSRS reporting, and more; making it a general purpose platform for any line-of-business applications. Developers can use their existing .NET skills to take advantage of various features of the Microsoft Dynamics CRM platform. This kit drills into the details of such development and explains how developers can connect, extend, and embed with the Microsoft Dynamics CRM platform. It includes presentations, hands-on labs and a VPC environment for practicing. This content was delivered earlier as part of the Metro Early Adopter initiative and the video recordings of the ramp-up presentations are included as well.
• CRM SDK Core (SDK – CodePlex)
  CRM SDK Core comprises a series of classes for the access to the data and the services of Microsoft Dynamics CRM 4.0. It comprises wrapper of the classes proxy for the web services of the CRM, support to Plug-in, classes of utility, classes of support to the complexities deriving from the various installations (Multi-tenancy) and from the various systems of authentication of the CRM.
• LINQtoCRM (SDK – CodePlex)
  The Linq to CRM project provides a custom query provider for Dynamics CRM 4.0.
• Microsoft Dynamics CRM 4.0 Forums (Forums – Microsoft)
• Microsoft Dynamics CRM 4.0 Videos on Channel 9 (Videos - Channel9/MSDN)
  Channel 9 hosts loads of useful videos. Here are just a few of them related to CRM Development (Click on this section header to see all CRM related videos) :
  Microsoft Dynamics CRM ISV Platform Overview 
  The Dynamics Duo talk about CRM and Silverlight
  The Dynamics Duo talk about CRM and WPF
  The Dynamics Duo talk about CRM and Office Business Applications (OBA)
  The Dynamics Duo talk about Dynamics CRM and SharePoint
  Creating a Silverlight control for CRM
  Creating a CRM WPF Offline Application
  Creating a custom SharePoint webpart for CRM
  Creating an OBA - Word Add-in for CRM
  Microsoft Dynamics CRM 4.0 Office Communication Server 2007 with Sanjay Jain
  CRM 4.0 Workflow with Praveen Upadhyay
  CRM 4.0 Deep Dive with Phil Richardson
  CRM 4.0 Overview with Phil Richardson
  Custom CRM Development with Aaron Elder
  ADXStudio: Marrying ASP.NET with Dynamics CRM
  Brad Wilson, Bill Patterson, Ben Riga: Dynamics CRM Platform

As usual, I mentioned the type of resource and the source after every link. The descriptions below the links are taken from the actual resource. They are not my personal opinion or comments. ;)

Coming up in the next part (part VI): Tools

In these series:
Part I: Introducing Dynamics CRM
Part II: Demos, Show Cases and Case Studies
Part III: Using Dynamics CRM
Part IV: Admin, Management and Deployment
Part V: Extensibility & Development (this part)
Part VI: Tools & Add-ons
Part VII: The Bits