What are other techniques are used to tenderize steaks apart from using a meat mallet?

Many marinades will act as tenderizing agents; particularly those that are longer-term or overnight.

The acids in the mixture will act on the meat to break it down and tenderize it.

Acids will help to tenderize meat. Most marinades include acids -- vinegar or citrus juice. You can also use tenderizing powder bought with spices -- it's made with an enzyme from papaya. Certain cooking methods (braising, or cooking for a longer time with some liquid in a covered dish) will make meat more tender. I've read that brining meat can help tenderize it too, but I haven't tried it.

There are the meat tenderizers that do actually stab through the meat with several blades.

http://www.amazon.com/Jaccard-Supertendermatic-16-Blade-Tenderizer/dp/B001343VTY

I wouldn't use it on a steak I'm going to throw on the grill, but I've used it with tougher cuts making cube steak for things like Chicken Fried Steak

http://en.wikipedia.org/wiki/Cube_steak

You can get best effect using syringe. Soda niter injections are very effective.

Instead of a mallet, you could use a rolling pin. The rolling instead of hitting will treat your meat more softly for a better result.

Marinating in oil. E.g. marinating rump-steak in oil is very effective to get a nice rump-steak.

In addition to acids (to which I would add Coke is a pretty good one), I've also had some luck with Whiskey. Leaves an extremely interesting flavor in the meat too.

Brining is my favored method. I regularly eat steaks for my diet, and I get the cheapest ones I can find. Brining turns them into restaurant quality.

Here's a great article on dry brining. To quote the scientific bit,

Salt is made of sodium and chloride ions that carry electrical charges. These ions attack the proteins, causing them to unwind a bit, a process called denaturing. These altered proteins have a greater ability to retain water, so meat that has been treated with salt remains moister through the cooking process.

Technically, it doesn't produce "similar" results to hitting it with a mallet, but it does get it soft and juicy if that's what you mean.

A lot of people know about wet brining (immersing your steak in salt water), but dry brining is easier and doesn't risk your steak getting waterlogged.

1. Dry your steak with paper towels or a cloth. Get it really dry.
2. Put about 1/6 teaspoon of table salt per kg. Simply put, it's a little more than you would otherwise would want on a steak. Some people suggest dry brining only one side, but I find that both sides work just fine.
3. Put it back in the fridge. Ideally, you'll want to put it the freezer on a cake rack or at least elevated a bit, to allow air on both sides. This is because the freezer has very little moisture in the air, so it'll dry up your steak.
4. Leave it in there for at least an hour. If you want to leave it in longer, I'd suggest putting it in the normal fridge where it wouldn't freeze up. I hear leaving it there overnight tastes better, but it's juicy enough after an hour.
5. Take it out. Keep it dry. Cook it.

Part of the beauty comes from how dry it is. The lack of water means no water vapor messing up your cooking. Your steak should also be moistened by the sodium. If you want to add sauces and marinades, cook it separately, though you can season it with herbs and spices. If you really want to cook your steak in water for whatever reason, brining will still soften it.