This is a tough one, but I think for my favorite mundane item is probably the ten-foot pole. It's easily the most classic item that all D&D groups need to have to go dungeon crawling. The thief thinks there might be a pit trap around and wants to check the floor? Ten-foot pole. The party wants to test the depth of a pool they find? Ten-foot pole. The party needs some extra firewood on the way back to town? Ten-foot pole. The thief gets the brilliant idea to pole-vault over a chasm to explore the tunnel on the other side? Ten-foot pole.
It's an item with dozens of different uses that every adventurer worth his salt should have.