I have just four wyandottes that are nearly impossible to tell apart. It's useful to be able to identify at a glance a particular bird I might wish to monitor, illness, injuries, bullying behavior, etc.
I'm frugal as well as remote from major feed stores. So I took colored, large size paper clips, straightened them out, and then curled them around a magic marker pen to create a spiral. To install them on the hens' legs was a bit of a chore, they put up quite a fuss and hollered that it was hurting, but I got them on without too much trouble. The bands are holding up just fine and it sure makes it easier to keep track of who'd who.