Custom Site Search is a WordPress plugin by BumpIn Team that allows visitors to search your website or the web from within your site/blog. This means that the user does not have to leave your page or even open a new tab/window. It uses google custom site search to search within your website/blog and shows the result in a bottom docked panel without making the site unresponsive. When the user clicks on any link in the search result the page is opened in new tab.
Download and info: http://wordpress.org/extend/plugins/custom-site-search/