Microsoft .NET Application Blocks

.NET Application Blocks are distinct pieces of code, created by the Microsoft Patterns and Practices Team, that demonstrate the best practices on how to accomplish a specific task using .NET. They are ready made code that you can use/extend to make your lives a WHOLE lot simpler.
I like them as they are extensively tested and reviewed (which hopefully minimizes security issues).

User Interface Processes Application Block v 2.0
http://www.microsoft.com/downloads/details.aspx?FamilyId=98C6CC9D-88E1-4490-8BD6-78092A0F084E&displaylang=en
The User Interface Process Application Block provides a simple yet extensible framework for developing user interface processes. It is designed to abstract the control flow and state management out of the user interface layer into a user interface process layer.

Smart Client Offline Application Block
http://www.microsoft.com/downloads/details.aspx?FamilyId=BD864EB5-56B3-43A5-A964-6F23566DF0AB&displaylang=en
The Offline Application Block, is intended to serve as an architectural model for developers who want to add offline capabilities to their smart client applications.

Authorization and Profile Application Block
http://www.microsoft.com/downloads/details.aspx?familyid=ba983ad5-e74f-4be9-b146-9d2d2c6f8e81&displaylang=en
The Authorization and Profile Application Block provides you with an infrastructure for role-based authorization and access to profile information.

Logging Application Block
http://www.microsoft.com/downloads/details.aspx?FamilyId=24F61845-E56C-42D6-BBD5-29F0D5CD7F65&displaylang=en
This block is a reusable code component that uses the Microsoft Enterprise Instrumentation Framework (EIF) and the Microsoft .NET Framework to help you design instrumented applications.

Aggregation Application Block
http://microsoft.com/downloads/details.aspx?FamilyId=9058F345-E5FE-42FC-B40B-14EBDD182F48&displaylang=en
The Aggregation Application Block is a .NET Framework extension that allows you to easily manage and coalesce information from various service providers and other systems and present that information to users.

Caching Application Block
http://microsoft.com/downloads/details.aspx?FamilyId=B55164C9-94C8-4077-AA29-AFE4074746DE&displaylang=en
The Caching Application Block has been designed to encapsulate Microsoft's recommended best practices for caching in .NET applications

Configuration Management Application Block
http://www.microsoft.com/downloads/details.aspx?familyid=85cb1c53-8ca7-4a92-85e3-e4795bd27feb
The Configuration Management Application Block is an easy to use mechanism through which you can read and write application configuration data.

Data Access Application Block v 2.0
http://www.microsoft.com/downloads/details.aspx?FamilyId=F63D1F0A-9877-4A7B-88EC-0426B48DF275&displaylang=en
The Data Access Application Block encapsulates performance and resource management best practices and can easily be used as a building block in your own .NET application. If you use it, you will reduce the amount of custom code you need to create, test and maintain.

Persistent Asynchronous Invocation Application Block
http://microsoft.com/downloads/details.aspx?FamilyId=794EC811-B5EA-46AE-BAA4-69A3DEADD38E&displaylang=en
The Microsoft Asynchronous Invocation Application Block manages asynchronous communication between a Web client and one or more foreign service providers (FSP).

Updater Application Block
http://www.microsoft.com/downloads/details.aspx?familyid=c6c17f3a-d957-4b17-9b97-296fb4927c30
In medium to large organizations, it is common to want to keep all instances of a desktop application up to date with the latest version of executables, libraries, and other files. The Updater Application Block provides an extensible framework that companies can use to create updateable applications

Exception Management Application Block
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=8CA8EB6E-6F4A-43DF-ADEB-8F22CA173E02
The Exception Management Application Block provides a simple yet extensible framework for handling exceptions. With a single line of application code you can easily log exception information to the Event Log or extend it by creating your own components that log exception details to other data sources or notify operators, without affecting your application code. The Exception Management Application Block can easily be used as a building block in your own .NET application.