The unknown Wraith beam was only seen once. This was when the Wraith were culling Orin's Planet. It is shown that a dart was using its culling beam and just after it went over an area a beam formed and started to go up into the sky. The purpose of this beam is unknown. Though seeing as it was on throughout the the rest of the culling after it was activated and caused no apparent damage it is most likely not a weapon. (SGA: "Letters from Pegasus")
beam fully formed