For a double rib, beginning with K2;
Knit two, pass the first knit stitch over the second knit stitch. [Bring the yarn forward. *Transfer the stitch from the right needle to the left needle. Purl two together. Repeat from * once. Carry the yarn back. ** Knit one, pass the first stitch on the right needle over. Repeat from ** once.] Repeat between  to the end of the row.
Very smooth finish, like a cut edge. I much prefer this more than the crochet-like edges on other cast-offs.
Let me know what you think.