What is this guitar playing accessory called?

It's a slide, they come in diffferent materials (glass, metal) they are used to give a unique sliding sound as you move up and down the neck. and yes it helps with barre chords as well.

http://en.wikipedia.org/wiki/Slide_guitar

that's called a digital postpone and there are quite a few kinds available with distinctive features, yet all of them fairly do an analogous factor, that's to repeat a quick sequence of notes or chords decrease back till you tell it to end. you will get one out of your community song shop or from any of the online merchants like Guitar middle, Musicians chum, Zzounds, etc.