My wife uses Lush brand things and I thought it'd be nice to store these upside down so that she could get the last little bit of the bottle. These work great.

There are two variations - snug and loose. Snug interferes a little with the cap so that it will stay on if the bottle is lifted, but it's pretty easy to remove. Loose does not interfere with the bottle, so if you pick the bottle holder up, the holder will stay on the shelf/rack.