Recently, Microsoft released their ASP.NET AJAX Framework which allows developers to build AJAX applications more easily.  ASP.NET AJAX consists of two pieces. 

1.) Microsoft AJAX Library - contains a set of script files that provide common functions and an OO programming framework

2.) ASP.NET 2.0 AJAX Extensions - includes a set of server controls that allows developers to add AJAX functionality by dragging and dropping controls onto a page

The following are brief descriptions of the javascript extensions made available by the AJAX Client Library by Namespace.

Global Namespace - contains members and types that extend base JavaScript objects.

Array Extensions (add, addRange, clear, clone, contains, dequeue, enqueue, forEach, indexOf, insert, pars, remove, removeAt)

Boolean Extension (parse - converts a string into a Boolean)

Date Extension (format, localeFormat, parseInvariant, parseLocale)
        var today = new Date();
        alert (today.format('D'));

        d - Short date pattern (05/10/07)
        D - Long date pattern (Thursday, 10 May 2007)
        t - Short time pattern (18:05)
        T - Long time pattern (18:05:12)
        F- Full date pattern (Thursday, 10 May 2007 18:05:12)
        M - Month and date pattern (May 10)
        s - Sortable date and time pattern (2007-05-10T18:05:12)
        Y - Year and month pattern (2007 May)

Error Extensions (argument, argumentNull, argumentOutOfRange, argumentType, argumentUndefined, create, invalidOperation, notImplemented, parameterCount, popStackFrame)

Number Extension (format, localeFormat, parseInvariant, parseLocale)

        p - number is converted to a string that represents a percent
        d - converted to a string of decimal digits
        c - converted to a string that represents a currency
        n - converted to a string of the form "-d,ddd,dd"

        var num = Number.parseInvariant("130.33");
        alert (num.localeFormat("c")); // $130.33

Object Extensions (getType, getTypeName)
        getType - returns the type of specified object
        getTypeName - returns the type name of an object

String Extension (endsWith, format, localeFormat, startsWith, trim, trimEnd, trimStart)

Sys Namespace

         Sys - root namespace containing some base classes such as Sys.CultureInfo
                  Application - objects/methods that expose client events and manage client components
                  ApplicationLoadEventArgs - container object for arguments of the Application Load event
                  CancelEventArgs - base class for events that can be canceled
                  Component - base class for all asp.net ajax objects including Control class and Behavior class
                  CultureInfo - object that can be used to provide locale specific functionality
                  Debug code - debugging and tracing functionality
                  EventArgs - used for storing event arguments
                  EventHandlerList - collection of client events for a component containing event names and handlers 
                  PropertyChangedEventArgs - contains event arguments associated with changed properties
                  StringBuilder - facilitates more efficient string concatenation

         Sys.Net - provides networking and communication support
         Sys.UI - contains set of classes for UI support
         Sys.Services - support for asp.net application services such as login/authentication
         Sys.Serialization - provides support for data serialization/Json
         Sys.WebForms - contains classes for async page loading