In addition to the well known VNC support in Sunstone, OpenNebula 4.12 will include support to interact with Virtual Machines using the SPICE protocol.
This feature can be enabled for any Virtual Machine just checking the option in the input/output section of the Template creation form.
This feature uses the SPICE HTML5 client, there is some functionality missing, but it is functional. For more information on this library, please visit the previous link. It also uses the existing novnc-server included in OpenNebula, as SPICE has no built in support for the WebSocket protocol