There may come a time in your golf playing that you have to putt with a club besides the putter. It might get damaged in normal play, or it might be the victim of frustration--as in, landing in a water hazard or getting snapped in half. The rules say you have to putt with another club to finish your round, and you're not allowed to borrow a putter from anyone else.
Try putting with your driver. Most golfers might think of the driver as the opposite of the putter because it gives you the most distance off the tee. But both clubs have relatively flat impact surfaces and the part of the club that strikes the ball is perpendicular to the ground.
Choke way down on the driver to putt with it. You need to control your putting stroke, but the length of the driver won't allow you to do that if you are holding it high on the grip.
If you don't want to try the driver, use a long iron, if you have it. Few golfers carry a 1-iron because it is so difficult to hit, but if you do, try it as a putter. It has almost no loft and the club's stiff shaft will help you keep the ball on track. Again, choke down on the shaft to get a controlled swing.
If the driver and 1-iron are out, use a 3-iron. It is the longest, straightest club in your bag. It has some loft, but that can be mitigated by angling the blade of the club so that it is standing straight. The problem with this is that you will have to play the ball back in your stance in order to stroke through the ball with a pendulum-like swing. Take two or three practice swings before you attempt to putt the ball with your 3-iron.
The sand wedge is one more option, if you are confident you can deliver a smooth stroke with it. While it has more loft than every club but the lob wedge, it also has a heavy leading edge. This edge can be used to putt the ball.
Tips & Warnings
Practice putting with other clubs from time to time.