The ASP.NET Framework gives you the most commonly used namespaces : Introduction « ASP.Net Instroduction « ASP.NET Tutorial

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2. ASP.Net Instroduction
3. Introduction

System
          System.Collections
          System.Collections.Specialized
          System.Configuration
          System.Text
          System.Text.RegularExpressions
          System.Web
          System.Web.Caching
          System.Web.SessionState
          System.Web.Security
          System.Web.Profile
          System.Web.UI
          System.Web.UI.WebControls
          System.Web.UI.WebControls.WebParts
          System.Web.UI.HTMLControls
          System.Web.Extensions
          System.Xml.Linq
          System.Data.DataSetExtensions

The default namespaces are listed inside the pages element in the root web configuration file 
located at the following path:

\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG\Web.Config          


Before you can use a class contained in an assembly in your application, 
you must add a reference to the assembly. 
By default, an ASP.NET application references the most common assemblies contained in the Global Assembly Cache:

          mscorlib.dll
          System.dll
          System.Configuration.dll
          System.Web.dll
          System.Data.dll
          System.Web.Services.dll
          System.Xml.dll
          System.Drawing.dll
          System.EnterpriseServices.dll
          System.Web.Mobile.dll