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

Shortcut	Description
$addHandler	Shortcut to Sys.UI.DomEvent.addHandler method
$addHandlers	Shortcut to Sys.UI.DomEvent.addHandlers method
$clearHandlers	Shortcut to Sys.UI.DomEvent.clearHandlers method
$create	Shortcut to Sys.Component.create method
$find	Shortcut to Sys.Application.findComponent method
$get	Shortcut to Sys.UI.DomElement.getElementId method
$removeHandler	Shortcut to System.UI.DomEvent.removeHandler method