Please find below some of the important links which have been collated on Microsoft Core .NET Platform.

.NET Framework Conceptual Overview
The .NET Framework has two main components:  the common language runtime and the .NET Framework class library.
The common language runtime is the foundation of the .NET Framework.

About Microsoft .NET 3.5 Framework
Microsoft .NET Framework version 3.5. Framework is an integral Windows component that supports building and running the next generation of applications and Web services. The key components of the .NET Framework are the common language runtime (CLR) and the .NET Framework class library, which includes ADO.NET, ASP.NET, Windows Forms, and Windows Presentation Foundation (WPF).

.NET Framework 3.5 Architecture
The architecture of the .NET Framework version 3.5 and 3.5 Service Pack 1 (SP1) builds upon the earlier versions of the .NET Framework.

.NET Framework System Requirements
 - System Requirements for Version 3.5
 - System Requirements for Version 2.0 
 - System Requirements for Versions 1.0 and 1.1

Official downloads
Microsoft .NET Framework 3.5 Service Pack 1
.NET Compact Framework 3.5 Redistributable
Windows Installer 4.5 Redistributable
Microsoft .NET Framework 3.0 Redistributable Package
Microsoft .NET Framework 2.0 Service Pack 1 (x86)
Microsoft .NET Framework Version 2.0 Redistributable Package (x86)
Public Sector Developer Weblog
Download the .NET Framework 3.0 Runtime Components
.NET Compact Framework API Documentation
ActiveSync 3.7.1
Windows Mobile-based Smartphone 2003 SDK
Introducing the .NET Micro Framework
Building Devices for Windows SideShow with the .NET Micro Framework
Porting the .NET Micro Framework 
.NET Compact Framework Downloads 
 

Resources

BCL Team Blog
http://blogs.msdn.com/bclteam/archive/2008/11/04/what-s-new-in-the-bcl-in-net-4-0-justin-van-patten.aspx

Community Blogs
http://weblogs.asp.net/communityblogs/archive/tags/Web+Services/.NET+Framework/default.aspx

Aaron Ruckman's Web Log
http://blogs.msdn.com/aaronru/archive/2008/01/19/installing-net-framework-3-5-language-packages-with-the-core.aspx

Wenlong Dong's Blog 
http://blogs.msdn.com/wenlong/archive/2008/09/07/net-4-0-wf-wcf-and-oslo.aspx

Wikipedia - .NET Framework
http://en.wikipedia.org/wiki/.NET_Framework

Architecture
http://en.wikipedia.org/wiki/.NET_Framework#Architecture

Standardization and licensing
http://en.wikipedia.org/wiki/.NET_Framework#Standardization_and_licensing

.NET Framework 3.5
http://en.wikipedia.org/wiki/.NET_Framework#.NET_Framework_3.5

.NET Framework 4.0
 http://en.wikipedia.org/wiki/.NET_Framework#.NET_Framework_4.0

.NET vs. Java and Java EE
http://en.wikipedia.org/wiki/.NET_Framework#.NET_vs._Java_and_Java_EE

Microsoft .NET new logo and branding
http://www.uxpassion.com/2008/10/new-microsoft-net-logo-and-branding/

.NET Framework Developer Center
http://msdn.microsoft.com/netframework/

.NET Framework Technologies
Data Platform Development using ADO.NET
 http://msdn.microsoft.com/hi-in/data/default(en-us).aspx

Getting Started with ASP.NET
http://msdn.microsoft.com/hi-in/asp.net/default.aspx

Base Class Libraries Community
http://msdn.microsoft.com/hi-in/netframework/aa569603(en-us).aspx

.NET Micro Framework
http://msdn.microsoft.com/hi-in/netframework/bb267253(en-us).aspx

Windows CardSpace
http://msdn.microsoft.com/hi-in/netframework/aa663320(en-us).aspx

Windows Communication Foundation
http://msdn.microsoft.com/hi-in/netframework/aa663324(en-us).aspx

Windows Client.NET
http://windowsclient.net/

Windows Workflow Foundation
http://msdn.microsoft.com/hi-in/netframework/aa663328(en-us).aspx

.NET Framework 4.0
http://channel9.msdn.com/tags/.NET+Framework+4.0/

Parallel Programming with .NET:.NET Framework 4.0 Poster for Download
http://blogs.msdn.com/pfxteam/archive/2008/11/03/9037931.aspx

Sandboxing in .NET 4.0
http://blogs.msdn.com/shawnfa/archive/2009/05/22/sandboxing-in-net-4-0.aspx

Tools and Utilities
.NET Tools and Utilities 
 http://www.c-sharpcorner.com/Resources/Tools.aspx

WPF Toolkit
http://sweux.com/blogs/smoura/index.php/wpf/2009/04/15/introducing-the-wpf-toolkit-datagrid/

What build tools are available for .NET?
http://windowsclient.net/blogs/faqs/archive/2006/05/26/what-build-tools-are-available-for-net.aspx

CLR Profiler for the .NET Framework 2.0
http://www.microsoft.com/downloads/details.aspx?FamilyID=a362781c-3870-43be-8926-862b40aa0cd0&DisplayLang=en

Fx Cop
http://msdn.microsoft.com/library/bb429476.aspx

Performance Console (PerfConsole) 1.0
http://www.microsoft.com/downloads/details.aspx?familyid=5a04ee30-6259-4fad-bf05-ccb72be09b4a

.NET Productivity Suite 
http://www.ideablade.com/downloads.html?referrer=MSDNForms

ILMerge
http://www.microsoft.com/downloads/details.aspx?FamilyID=22914587-b4ad-4eae-87cf-b14ae6a939b0&displaylang=en

LibCheck
http://www.microsoft.com/downloads/details.aspx?familyid=4b5b7f29-1939-4e5b-a780-70e887964165&displaylang=en

CLR Profiler
http://download.microsoft.com/download/a/b/d/abd115c6-a96e-4d8d-b7de-9480deb6cd5c/clrprofiler.exe

Windows Communication Foundation Tools 
http://msdn.microsoft.com/en-us/library/ms732015.aspx

Windows Presentation Foundation Tools 
http://msdn.microsoft.com/en-us/library/ms742404.aspx

Windows Workflow Foundation Tools
http://msdn.microsoft.com/en-us/library/ms734408.aspx