What is your favorite piece of backpacking gear (in any category) that you don't leave home without?

[greengirl]

I don't leave home without one of my Xped sleeping pads (summer or winter version) b/c it is the most comfortable I have personally found. It allows a good night rest for those 20+ mile days.

[SILHiker]

It would be a toss-up between my Big Agnes air core pad, the MSR pocket rocket 2, and my TOAKS 650 ml pot.

[Rob6]

The one thing that I never leave behind is a knife. At the very least, a sturdy EDC pocket knife; or a small fixed-blade bushcrafter knife.  To paraphrase @Tetons2 , it is just so versatile!  I've used mine for everything from splitting campfire wood to carving up a freshly picked paw-paw for an on-trail snack!

[Philreedshikes]

(can you have many items as favorite?)  Anyway...if my air mattress blows, trip is over for me!  I need to stay warm and dry...and recommend not leaving the trailhead without the "ten essentials"!

[RedPackWalking]

Besides the essentials and my trusty trekking poles, three pieces of gear I've used the past couple of years that I'll NEVER backpack without are (two are girl-specific, so sorry guys):

• My pStyle FUD (female urinary device) - it allows me to pee standing up and I don't have to drop my drawers every time I have to take a leak. I LOVE IT!
• My Brondell GoSpa Travel Bidet - I don't use toilet paper on the trail anymore (I carry some as a backup in case I'm camping somewhere there's no water, and I pack out the used stuff if I must use it) and it gets me fresh and clean! It is incredibly useful during that time of the month too. 
• My Diva Cup - I don't have to carry tampons and pack out used ones, and I can go 12 hours without having to deal with my period at all. The above-mentioned item comes in handy to deal with cleaning up. 

LNT baby! 

[awalkoutside]

Whether it be an ultralight foam pad, or my REI Flexlite chair I never leave home without something to sit on!

"From the highest king, to the lowest peasant...everyone enjoys a good sit."