I use Boomerang Tags -
Pet tags, dog tags, pet id tags, dog id tags, pet identification tags, cat id tags
They're relatively inexpensive, DEEPLY engraved so the info never wears off or becomes illegible... and they're guaranteed for the life of your pet. I have habging tags, CollarTags, plastic tags, and the brass bolt-on tags from them... all are of superior quality, they're amazing.