Javascript is a great language for client side scripting. Most modern browsers support javascript so unlike flash or Silverlight the user does not need to download another plugin to see Javascript applications.
Silverlight, Flash and Javascripts are all great technologies and has there own advantages as well as disadvantages.
Ajax which stands for Asynchronous JavaScript and XML is a great technology which enables developing rich web based applications.
Javascript community has built some very cool Libraries to aid developers and one of my favorites is JQuery.