While there are many imitations, true 550 "para-cord" or military shroud-line is hard to beat. This rope, often sold in a spool of 1000' or in bundles of 50', is a survivor's best friend. The seven inner-strands each have a tensile strength of 30 pounds and can be stripped out. We have used these inner strands as fishing line, in netweaving, as stitching, and in netted containers. I once used the exterior sheath of 550 to weave a small hammock. (I had some time on my hands.)
Kept intact, 550 cord is my primary shelter-lashing material when making lean-tos, wickiups, and tarp-shelters. Despite its great tensile strength, I wouldn't rely on this for rappelling as some survival books suggest.
You can find spools of 550 at Cabelas. They usually come in olive drab or brown. One instructor I know buys his para-cord in a white color and then dyes the rope orange so it shows up better in the wilderness. Be aware that some hardware stores carry their own version of "para-cord." This is not true 550-cord and has a weak, cotton interior. It is intended for clotheslines and won't last more than 5 minutes on a tarp in high-wind! Find the good stuff and then carry several 25' bundles in your pocket, pack, and vehicle. Sure you can weave rope from plant fibers like our ancestors did, but that's not what you want to be doing when the chips are down in a survival setting.