Ajax links API module help you to load any content to a specified DIV or ajaxify whole drupal site, using JQuery Ajax. This module will also load JS for Ajax-loaded content by using Drupal.attachBehaviors(). In case you dont want to load js or css, you can alter tpl for path/node type.
Links will work as normal if Javacript is disabled so no effect on web crawlers used by search engines.In HTML5 browsers, title bar and url will change using history.pushState(). Also, it will update active class on links.