Correction:
There is 1 free option - you must own an iPhone or iPod.
There is one inexpensive option at .99c but you must own one of the rather limited number of cell phones supported and your wireless plan will be billed the .99c.
These is one full price option at 6.50, the mobile authenticator through Blizzard.

Any way you do it, it's a great idea.

There is 2 free options

1- Ipod/Ipad/Iphone

2-Android on android cellphones and windows android emulators

I leave to you the pleasure of googling for that.

@ducss750 Correction to your correction:
There are 2 free options. iPhone/iPod or Android both have free authenticator apps. As an Android user, I should know ;)

Give me a iPhone / Android and then I'll have a free option ;)

I already have an authenticator that I ordered. I love it. However, now I bought a Droid (evo). Can I download the Droid version and use both? Or would the original need to be unlinked?

Edit to my much-corrected post:
There is 1 free option - you must own an iPhone, iPod or Android based phone.

Consider the deceased equine suitably flogged :)

(For those of us who use a smartphone with *another* os, we are out in the cold. My nephew is getting a keyring authenticator for Christmas, I ordered mine at the same time)

@ welan
No, just like Highlander "There can be only one"

From Blizzard:
Can I keep one Battle.net Authenticator at home and another at work, and have both associated to the same account?

No. Each account can have only one Battle.net Authenticator linked to it at a time, so you would need to carry the Authenticator with you to log in from different computers.

I'm sorry, but unless you are in a country that cannot get an authenticator for various reasons , there is no reason why players should not have an authenticator. Just like there is no reason no one should not have a smoke/fire alarm in their house or seatbelts in their car.

yeah yeah yeah, everyone is so careful and smart and canny and no one can ever hack them because of their total uberness. Got it.

$6.50 is the cost. Unless you are dependent upon a parent or a special ed counselor for money, there is no reason to not have one, if you can get one. It's that simple.

And any Guild that allows bank access to someone without one is a poorly run Guild , IMO.