A tiny little keyring torch cost about 3 quid off amazon. It's actually touted as a light for a dog collar - and I don't have a dog!. I have it attached to the small strap inside my ruckask, at the top, that is designed to hold the H20 tube (I don't use H20 bladders). The light comes in handy for seeing to the bottom of my rucksack, even in daylight. It can be a pain in the valley to find something quickly sometimes. like something small that might have dropped to the bottom of the bag.
A cut up rectangle of an old fitness mat. Slips inside the H20 sleeve. I use it as a sit mat and it also helps gives some shape to my rucksack if its fairly empty.
An old zip grip I took off an old football boot bag zip. it has a rubber 'U' shaped end and is really easy to grip. Attached it to the main zip of my main rainjacket so I can zip it fast even with winter gloves on.
A small cloth-towel with a small loop on. It is as small as a hankerchief I guess. Got it from Ikea for about a quid ages ago. Used it in many different ways. Attached it to a loop on the inside of my waterproof bottoms. If I need to dry something quicky when its raining, a quick unclip and hey presto. Also has served as a wiper for my inner tent if there has been any condensaton. Or for the outer if it has got a bit mucky and needs a wipe up before packing away.