I’ve seen a number of people having a favourite nonmagic item, and in many ways I think that this one of the best parts about D&D, creatively using simple items. Darths and Droids mentioned this yesterday, actually (http://darthsanddroids.net/episodes/0636.html).
Gotrek, the notorious dwarven warlord from the Kaldahar Chronicles I’ve been posting up, had an unhealthy obsession with the portable battering ram. Not sure how well it was used, but it was certainly used often. The series of assassins played by another friend of mine of course found the grappling hook to be essential.
|From the WOTC website. Its the one that looks like it would tangle your feet.|
As for myself, there is one item that has recently leapt ahead in my esteem as the most useful nonmagic item in the game. Though I would normally cite Samwise Gamgee’s recommendation of rope as the essential item for any adventure, I have to revise my opinion and put forward the Tanglefoot Bag as my favourite item, because this is the item which let a level 6 party completely annihilate a powerful lich in his own tower. In an encounter which was intended for our party of thieves to sneak past the wizard and steal his thing without going anywhere near him, the DM managed to push all of my characters buttons at once, and so I rushed forward, dump tackled the undead monstrosity and threw a tanglefoot bag on him. A wizard on the ground, surrounded by fighters and covered in goo is done for, no matter his level.
I never go without a tanglefoot bag anymore. Several if I can afford it.