The other permanent option, tattooing, is not generally offered in my area. Also, tattooing anywhere but the ear has to be shaved to be read.
The third option, of course, is a collar tag. Granted, cats tend to lose their collars, but mine wear harnesses instead. One of our former cats had a metal tag with her name and our phone number on her harness. She escaped out the door while I was unlatching her leash, and was gone for two days. I got a phone call on the second day from a lady in the apartment complex down the street. She had spotted Lydia and read the tag on her harness, then called me. I went down the street and got a very scared, but happy, cat.
Even if your cat wears a collar instead of a harness, I would recommend a collar i.d. tag. If the collar comes off, it can be picked up by someone and read. Then, you have an idea of where your cat has traveled. If your cat is found with the collar still on, then he or she will be returned to you as soon as you can meet with the people who found your pet.
The Together Tag is not just a collar tag, however. It offers some of the services that a microchip does. It is a pet recovery service, a lost-and-found ad, and it contacts the local shelters about your pet. The tag has a unique code on it, and a page on togethertag.com which gives the finder information to locate you and any special requirements for your pet (diet, medications, etc.) while waiting for you to reclaim him/her. The information you submit can only be read by someone who has the pet's unique code, and you can put as much or as little information as you think necessary on the page. You can even include alternate addresses and out-of-state contacts, for use during an emergency or a national disaster.
The tag is also the cheapest recovery system offered to this date, priced at a one-time fee of $24.95, and tested under many environmental conditions (including disaster-related conditions like fire and salt water).
Click here for more information or to buy a Together Tag. For each tag purchased, $5 goes to the Red Cross.