Pill bottles are super light, free (after you pay for the meds in them, of course) and water-tight. I hesitate to declare them waterproof, because they may leak at some point, but in all my testing, I've yet to have leaks develop. All of these notions are based on the default clear amber bottle, roughly 1" in diameter, but some of the larger bottles can open up even more interesting possibilities.
Match Case: The same dryness that protects and preserves your tinder will do the same for strike-anywhere matches. If you glue a small square of the striker material to the inside of the lid, you'll also have a ready surface to ignite your matches against.
First aid kit: I've built countless toss-and-go first aid kits in pill bottles. The basic bottle is big enough to hold a half-dozen adhesive bandages, a couple alcohol swabs, and a couple packets of ibuprofin or aspirin. Bigger bottles can hold even more. It's not a complete kit, by any means, but it's actually very handy (say, for a mom of two active boys. Ask my own mother about this, and where I might have learned it from.)
Emergency fishing kit: Wrap fishing line around the outside of the bottle, and secure it with a pass of duct tape. Put a selection of hooks, weights, swivels, and possibly a lure or two inside the bottle. It's not pretty, but it's far better than nothing, and with good lure selection and a bit of practice, can catch fish quite effectively. Put a small piece of cork or foam rubber on the barbs of your hooks to protect them and to prevent a sharp metal snag from appearing inside your kit.
Sewing/patch kit: A couple needles and pins, a couple squares of cloth common to your clothing (mostly denim for me, but other wardrobes obviously differ), and a yard or two of thread in a couple common colors, and suddenly a snag or split is an inconvenience instead of a potential crisis. Add in a small selection of buttons if you have some available and wear clothes that button up. If you wrap the thread around the index and middle finger of one hand, it will fit neatly and smoothly into the kit.
Change holder: There are still pay phones out there (Google it, kids), and they still require change. In addition, a spare $10 or $20 folded up alongside can come in very handy when you're in need.
Jewelry/"Stuff" holder: My wife swears by small bottles to hold her earrings, rings, and other small accessories when camping. I keep my ear protection for shooting in one.
So, what's in your bottles?