OK, I love, Love, LOVE these arm warmers! I made them yesterday using these steps and knee-high socks. (I didn't hem mine on the ends, as they are knit and won't unravel.)
Arm warmers are perfect for those 3/4 length sleeves that don't give quite enough coverage on a cool day. Or coat sleeves a wee bit short. Arm warmers fill the gap! Also very cute with the sweater vest look, as this crafty gal pointed out at Angry Chicken. And I wish I knew how to knit like Philigry!
If you're looking to buy, rather than make, check out the great ones here. I'm lusting over a couple of these!