Have you ever worked with Microsoft Application Blocks, if yes then which?

Application Blocks are C# and VB.NET classes distributed as Visual Studio projects that can be downloaded from Microsoft's Web site and used in any .NET application, including ASP.NET Web applications. They are useful and powerful tools that can make applications more maintainable, scalable and efficient
Secondly which application blocks has been used depends on really what you have implemented. But there are two famous MAB which is making buzz around the industry:-
ã data access block
The Data Access Block provides static methods located in the SqlHelper class that encapsulates the most common data access tasks performed with Microsoft SQL server. If the term "static method" is new to you, it means that the class methods can be called without instantiating an instance of the class. For example, the method ExecuteReader () within the SqlHelper class can be called by simply using the statement SqlHelper.ExecuteReader () -- no object instantiation of the SqlHelper class is required.
ã Exception management block.
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
Note: - Itfs beyond the scope the book to look into details of application block. Best is go to www.microsoft.com and search for these application block. Try to compile one or two programs using their given classes and documentation.