Traditionally, it's a sabre. One of my wife's colleagues, a French lady, learned in secret how to do it, and the scared the shit out of everyone at her wedding reception - especially her new hubby - by whipping out a sabre and opening a few bottles of bubbly.
In actual fact, it's not as spectacular as it sounds. You don't swing the hardware around. You slide the blunt side of the blade up the bottle and hit the weak spot where the seam meets the top part of the bottle. With sufficient force the top of the bottle comes off, with the cork still inside it. You lose a bit of champagne, and everyone should check their drinks for shards of glass.