I heard about this gadget while listening to the Math For Knitters podcast (I no longer listen to it - the poor audio quality started to bug me).
It's called the Ablet and it's seriously amazing. (No, they haven't paid me anything to tell you this.) I bought one for my mom for her birthday, and she leaves it clipped to the handle of her knitting bag. I wear mine on my right hand when I'm knitting, and it has completely changed my accuracy and pattern counting. Nevermore shall sleeves be different lengths (phew).
If I'm doing something with pattern row repeats, I use the white beads to count my rows, and the brown beads to count how many pattern repeats I've done. No more pencil and crumpled paper (or calling out, "Husband, please remember the number 17 for me!"). Otherwise, I just use it as a normal abacus (white for the 1s column, brown for the 10s).
So. Much. Better.
The only problem is when I get very engrossed in the knitting and forget to move the bead. That's not a design flaw, though, it's operator failure.
Slightly amusing story regarding my Ablet: I was using it to knit on a train journey up to York for a work trip. When we were about 10 minutes from York station, I started packing up my things, and went to take off my bracelet. The chap sitting next to me said, "York's quite safe, you know, you don't have to remove your jewellery." Erm, thank you, sir.