We propose a simple experimental technique for visualizing Lagrangian coherent structures (LCS) in turbulent free-surface flows. The technique employs digital photography to record the transport of passive tracers (small paper pieces) introduced manually at the free surface. Coherent eddies are detected by time-averaging the instantaneous light intensity fields on finite-size temporal windows. We demonstrate the potential of the method by applying it to visualize the flow in the vicinity of a surface-piercing rectangular block mounted at one corner of a rectangular open channel. We show that by appropriately selecting the time averaging window, the technique can reveal the presence of organized patterns in the chaotic instantaneous flow and elucidate their complex interactions.