If the sheer number of characters I have played throughout various systems and games that have owned this item is any indication, my favorite nonmagical piece of kit would have to be the grappling hook.
Though sometimes weighty and/or bulky, the grappling hook is an invaluable piece of gear for it's usually light price tag. Not only can it be utilized in the primary functions it is designed for, such as pulling ships in for boarding or hooking parapets to climb towers, but the grappling hook can also be used as an anchor if it can catch a reef or tree (for airships, for instance), as the pulley mechanism for a harness if the ropes are strung correctly, as a hunting/fishing tool, to help wrangle monsters (especially those that fly), and as an improvised weapon.
The grappling hook is the type of item that rewards cleverness and ingenuity, as it only has as many uses as one can think up.
Plus, it just seems a natural fit for my usual character type - the Rogue.