This article has a good start on reuse of those pesky empty pill bottles. I find most of mine have about 10% of the space filled, the rest is air. I combine the left-overs of the same medication at month end and either reuse or recycle the bottles. Some of his ideas are new to me -- like a sewing kit. I'll work on one of those soon in lieu of the little hotel kit in my BoB.
Here are some of my favorite uses, most of which are for reducing clutter or adding a layer of water resistance to whatever is inside:
LIGHTERS: 3 of the BIC mini-lighters will fit in one, reducing the potential for accidental discharge of the butane.
CAN OPENERS: I carry 6 of the larger P51 can openers (3 for me, 3 to trade/give away) in one of these bottles in my BoB and another in my car kit.
FIRESTEEL: KMart has those caps that are child proof if used one way and not if turned upside down. The Upside-down mode makes the bottle just long enough to fit a small firesteel.
MAGAZINES: A medium size bottle (usually for bigger pills) holds 3 magazines for my small pistol. There may be a size that will hold rounds or magazines for your small pistol. Obviously not your first source of a replacement mag in a pinch.
WATER Purification: See below
KMart (yes, I love my KMart Pharmacy) occasionally has colored tops. I usually ask for red or blue ones. RED (or pink) for anything related to fire (matches, lighter, firesteel) and BLUE for anything related to water purification (tablets, small vial of 10% iodine solution, wad of clean muslin or coffee filters). That makes these important provisions even easier to spot when your are cold or setting up for cooking.
So why these used bottles and not just some little ziplocks? The pill bottles are less likely to be crushed in a tight pack, or to be damaged by the content -- like the can openers punching holes and falling out of the bags. The caps are more reliable than a zip lock top, especially if your hands and the plastic bag are cold. The filled bottles are relatively small, but are large enough not to become part of the lint, dust and crud that collects in the bottom of your pack.
Needless to say, I don't recycle many of these pill bottles anymore.