BlaB! AX is a free AJAX/PHP group chat system. Both the frontend and Admin CP are with responsive design, HTML5 and CSS3 compliant, support all modern browsers (incl. mobile browsers), HiDPI displays ready (no raster images).
BlaB! AX is tiny - all that users load with their browsers is less than 200kB (60kB gzipped) in 5 HTTP requests (incl. sounds, emoticons and other images; stickers and avatars are loaded on request) and 95% of this content is cached for further use.