What is a must-have iPod/iPhone/iPad accessory?

Airport Express. Without a doubt, gotta have it. It features "Air Tunes" which let's you play your music from every speaker in your house at the same time (provided you have multiple Airport Express units). And with that, the free Apple Remote app for the iPod/iPhone/iPad which can be used with iTunes on your computer and Airport Express to control all the music in your home.

If you are a photographer, and you have an iPad, than the Camera Connection Kit by Apple is a must have for photographers that are on the go. The kit has a pretty good price at $29.00, and (not saying that I support jailbreaking) if you jailbreak your iPad, you could use the USB port as a real USB port other than using it only for photos. Once again, I do not support jailbreaking.

I think that for every iPod/iPhone/iPad, you need to have a good case that's on it, because all it takes is one drop, and you might not have one anymore.