Guacamole

Guacamole is an HTML5 remote desktop gateway.  The application provides access to desktop environments using remote desktop protocols like VNC and RDP. A centralized server acts as a tunnel and proxy, allowing access to multiple desktops through a web browser.  No browser plugins are needed, and no client software needs to be installed.  The client requires nothing more than a web browser supporting HTML5 and AJAX.

http://guac-dev.org/