We've used Guacamole in production with a custom frontend. It's nice to work with and most problems I've encountered were covered in the old forums on SourceFor

We've used Guacamole in production with a custom frontend. It's nice to work with and most problems I've encountered were covered in the old forums on SourceForge. For anyone planning to implement a custom frontend I'd recommend using WebSockets exclusively. We've had trouble with XHR based connections.