I use identiTAG - the writing doesn't fade and you can put a lot of info on the tag
http://www.identitag.co.uk/Petidtag/index.htm

I once found a collar and tag in some woods. I phoned the lady up to tell her i'd found them and then went and posted them through her door. I hope I get the same in return if any of mine ever lose their collars or tags.

I use embossed ones as they are the only ones which can withstand the dirt, water and tough vegetation. I have surname, full address, mobile number, vets phone number and the words "microchipped" and "neutered" on there.