From my recent weekend outing...
Safety pin -
holdup a bug net (attach thru the seam)
Hang a mesh bag from your ridge line.
attach a bandanna to your hat for sun protection.
Peanut butter jar (empty plastic Jiff container) - carried in first dinner meal in the jar, put in boiling water, screwed back on cap and let it re-hydrate/cook. used the jar all weekend for re-hydrating, cooking, dipping stream water to boil, hot drinks, and packed out trash afterwards. very useful, might be my new favorite cheap cup/mug. and the lid is watertight.
Binder clip - (small spring steel for holding papers together)
Cinched door closed on tarp during a rainy spell
Held sleep/sit pad in place on tyvek groundsheet
Prusik knot - always useful to know the right knot for the situation (tarp on ridge line)
Fleece stuff sack - when stuffed with clothing makes an awesome pillow. - I like this so much I might actually make a 1/2 fleece - 1/2 nylon stuff sack for this very reason and cut down on some weight.