By default ASP.NET 3.5 Web App project has references to assemblies System.Data.DataSetExtensions.dll and System.Web.Extensions.dll.
With System.Data.DataSetExtensions everything is clear for me, it contains such useful extension methods as DataTable.AsEnumerable() and DataTable.CopyToDataTable().
And what about System.Web.Extensions.dll? What useful members could you mention to use?
System.Web.Extensions is not about extension methods like System.Data.DataSetExtensions, it contains the ASP.NET AJAX Framework.