That's a tough call. Right now, I'd say the least-used piece of kit (counting only items of significant cost, so that eliminates things like a ColorRight WB filter, the spiked feet for my RRS TVC-33, etc.) would be my PocketWizard setup - MiniTT1, pair of FlexTT5's, AC3, and a PowerMC2.
Generally, if I'm not using it and don't plan on using it in the future, I sell it. Putting my money where my mouth is (or rather, in my wallet where it belongs), I'm sitting in a Starbucks finishing my mocha after selling a couple of surplus Wimberley P-20 lens plates to a Craigslist buyer 5 minutes ago. I had planned on keeping one of them (I don't
plan on buying the new 100-400L if it shows up, but who knows?!). However, I've got RRS L84 plates instead of the Wimberley versions for my 70-200 II and 70-300L, and they're a bit lighter and have a 1/4"-20 socket (which came in handy once), so if I do get another lens which needs that size, I'll just order the RRS plate.
To that end...at some point soon I'll take some product pics of the PW set, and sell that.
Of gear that I intend to be part of my active kit....EF 40 mm f/2.8 pancake.
I use mine fairly frequently. Often I find myself going to an event expecting to be a bit distant from much of the action, and bringing the 70-200 II. I could bring the 24-70 II, but that would mean wearing the lens on a belt pouch or going back to a backback left at a seat. Instead, I slip the 40/2.8 into a pocket - the camera + 70-200 is on a BR strap connected to the lens plate, and if I need a wider AoV than 70mm, I just leave the telezoom hanging from the strap and use the pancake lens.
Photo backpack. It just sits in the closet because it's too cumbersome to shoot out of, and I never find myself in a position to carry everything I own. Interesting - that's probably the one piece of (non-Canon) gear I use the most. I have a shoulder bag for street / studio shooting, but my Lowepro Flipside 400AW is my constant companion when I shoot outdoors.
I lost you at backpack. Singular. Maybe any one of my many backpacks and bags would fall into the 'least used' categoty by itself (probably the Lowepro Toploader Pro 70 AW, since that works with the 70-300L but all my other body+lens combos fit in the shorter 65 AW or the longer 75 AW). That's why I have so many bags/packs.