Totally free and virtually weightless.
1. A empty Gatorade bottle: I would rank this below only my sleeping bag as essentials for trail enjoyment. Oh too many times I got up out my tent or shelter on a bitter cold night to take a pee. It took a while to figure out the Gatorade bottle thing, but it made me much happier on the trail. I'm so addicted to this technique I now utilize it in everyday life. Note: this may not be applicable for women.
2. Handkerchief: I used an old t shirt and cut into two squares. One I used as I sign to hitch rides with, one side saying "To Town" the other "To Trail"
The other rag I used for cleaning out pots, and general wiping of stuff.
3. Condiment packets: I loaded up at the Vince Lombardi Memorial rest area on the NJ Turnpike. I found the Hot sauce and the spicy mustard packets added zest to my usually boring Ramen. The salt and pepper packs are good too.
4. Trash Bags: a guy handed me two when I was getting on the bus to GA. They were handy. One was a layer of waterproofing for stuff in my bag, the other an emergency raincoat for the warmer dryer months when u use a raincoat never, as it can make u too hot.
5. Safety Pin: steal from your mom, wife, gf, or a niece or nephew. Temporary fix for clothes, tent and great at popping blisters!
I'm sure there are more. But these help me ongoing.