Since I've been knitting i-cords recently for the first time for a while, I thought I'd post about a couple alternatives -- a twisted cord and a crocheted cord.
The twisted cord is pretty self-explanatory. To make this, get several strands of string or yarn, anchor one end, and twist the yarn/string from the other end. Bring the ends together, and let the strands twist around each other. Finally, tie a knot where the ends meet to prevent the cord from untwisting.
The other cord, a crocheted cord, is a lot like a knitted i-cord. Here's one way to make the cord:
Make a ch-4 loop.
Rnd 1: sc 6 into loop.
Rnd 2, etc: sc once into back loop of each sc (6 sts)
In other words, start out as if for a spiral hotpad. Then crochet the hotpad as before -- except do not make any increases. Each round consists of 6 sc's. In variation here, stitches are crocheted through back loops only. Crocheting into the back loop (as opposed to crocheting under both loops as we have in every pattern so far) leaves a ridge which spirals around the cord.
The cord in the picture is made in Peaches & Creme and a size 7 (4.50 mm) crochet hook. It's about 3/4 inch in diameter. -- A knitted 6-st i-cord would have a large diameter as well.
To make a narrower crocheted cord (that looks more like a 3-st i-cord):
sc 3 in 2nd chain from hook.
sc in each sc until cord is desired length (3 sc's per round).