I haven't seen my favorite item of all time here. I notice lots of bits about rope, about grappling hooks, and about other similr items, but you've forgotten the single magic item tht does anything you could ever need from ropes OR grappling hooks.
Complete Scoundrel gave us: The Rod of Ropes.
You want to fire the grappling hook, but don't have the Use Rope or Climb to scale it? It does that. It pulls you up by itself, and it does it at 60 feet per round. You need a whole lot of rope, and fast? It does that. Need to slide from one place to another? It has you covered. Badguy perched precariously next to a fall and you aren't standing next to him, or lack the manly fortitude needed to heroically punch him? Yeah, this puppy can bullrush, too. Or heck, stand in the middle of a building and knock two people off! If a chicken lays an egg on the middle of a roof, which way does it roll? Whichever way you want it to. Did I mention the grappling hooks have a 300 foot range - on both ends? You might as well attach one end to the moon after it's thrown by a hulking hurler, the other to the earth, and just slide all the way down through space with that range! And it throws all 300 feet in a single round! While the wizard is futzing around with his spells or the fighter is taking off his boots to put on his dainty little slippers, you've already scaled the cliff and are hocking loogies at them!
Rod of Ropes is more or less half of Batman's gadgets - you know, the ones that used rope - and the hookshot from all the Zelda games combined into one. One might claim the cost of 4,000 gold is too much...but I've used it in just about every session of every game that I've been in.