When I was with Happy Camper Studios we used to stick colored index cards up on our desk backboards or atop a monitor to indicate willingness to be interrupted.

When I was with Happy Camper Studios we used to stick colored index cards up on our desk backboards or atop a monitor to indicate willingness to be interrupted. Red meant leave me alone; green meant open for interruption; yellow meant you can poke me if it's work-related. We looked around for lights and such (I wanted a traffic light) but they were too pricey so we went low-tech.