Of the most basic devices, as we pointed out in earlier pages, are the belay tubes, or plates. The simplest, most boiled-down mechanical device that there is, really, is the link of chain, or the friction saver ring.
Just to exhaust the simplicity end of it, and since newguy was going to give the chain link a try, here is a picture I took. I actually did a climb with this in 2:1 Ddrt mode and SRT. Here is a picture of it in SRT mode:
