There is a bunch a great info here. I'll use the suggestions to beef up my first aid kits, (one on the road bike & a second for the mountain bike).

One of the items that I haven't seen mentioned, but I use a lot, is a little tin of salve. I've used it to keep a bit of road rash comfortable during the remainder of a ride. The salve will even stem minor bleeding. It works as a balm for wind chapped lips & cheeks. And, it has come in handy to clean chain grease off my hands with something slightly abrasive like a tissue.

Another item that I keep in my kit is a $20 bill in case I need to buy other emergency supplies during a ride. I used to carry my debit card but frequently forgot to return it to my wallet after the ride. Carrying cash saves me from having to remember the card.