Perhaps you just followed the steps in the Deploy Office Web Apps Server Preview and the Configure SharePoint 2013 Preview to use Office Web Apps Preview TechNet articles and now when you try to test if your installation succeeded you receive an error. If you see the following error in the ULS logs the solution is simple! Just free up some more memory and you are good to go. It just worked like a charm for me.
WOPIFrame – Unhandled exception: System.ServiceModel.ServiceActivationException: The requested service, ‘net.pipe://localhost/SecurityTokenServiceApplication/appsts.svc’ could not be activated. See the server’s diagnostic trace logs for more information.