Use the "Ajaxize" plugin found in the WordPress.org Plugin Directory (wordpress.org/extend/plugins) if you need a comprehensive AJAX solution that enables AJAX technology on any WordPress PHP function, including core functions, a custom function you wrote or other plugins you've installed. The "Ajaxize" plugin does require a bit of work, as you must enable custom functions automatically through the plugin's settings. The plugin has a five star rating in the WordPress Plugin Directory.
The best way to control your WordPress site with AJAX is to install one or more AJAX-based plugins. These types of plugins enhance your site with AJAX features, which control how your posts, pages, comments, archives or other data are loaded. The content is loaded asynchronously by calling to the Web server, requesting the data and then displaying back the content in the same page view without reloading the page. AJAX plugins can also speed up the performance of your WordPress installation.
Install the "AJAX Read More" plugin, also found in the WorldPress Plugin Directory, to convert the standard "Read More" links to display the remainder of the blog post on the site's front page. The plugin calls the entire post into the browser's memory when it is loaded, but only displays the beginning part. When the reader clicks the "Read More" link the bottom half of the post is shown. This AJAX-based plugin also has a five star rating in the plugin directory.
Use the "AJAX Comment Posting" plugin if you would like to add AJAX functionality to your WordPress comments. A reader's comment is posted under a blog post without refreshing the page. The plugin also checks to see if the comment form was filled out correctly by the user. If it was not, then it alerts the user of any errors before it lets the comment go through. The "AJAX Comment Posting" plugin has a simple installation process and doesn't require further customization or settings. It works right away after installed.