The energy beam with which a Borg cube slices sections out of another vessel's hull for study and assimilation, first turned on the Enterprise-D at System J-25.
The cutting beam is one of the standard tools of a Borg vessel. Usually applied after a tractor beam has seized the target and drained its shields, it cuts cleanly through hull plating so that part of the ship can be removed and examined.
In the Next Generation episode Q Who, the first cube the Enterprise-D meets carves a cylindrical core out of the saucer section, taking eighteen members of the crew with it. In The Best of Both Worlds the Borg begin cutting into the engineering hull before the ship breaks free and hides in a nebula.
The beam is seen again in the attack on the USS Saratoga at Wolf 359, shown in the Deep Space Nine pilot Emissary, and in Star Trek: First Contact.