A basic metal tag with my name, city and cell phone number (city is there because I have a non-local area code on my cell and don't want someone to get confused as to where he is from). When I go on vacation, I'm getting another one engraved (self-service kiosk at the pet store) with my parents' cell phone numbers and home number since I won't have my cell with me.
Don't go too high tech. You want anyone and everyone to be able to read the tag. The USB ones, the QR ones etc aren't going to get looked at by the majority of people. If you worry about the tag coming loose, there are good quality ones that slide over the collar or some that rivet on (for leather collars especially). You want a 24 hour contact number or preferably, 2.
And microchip too!