I've been using quarter-inch electric rope a lot, like this:
Amazon.com : Baygard Electric Fence 1/4-Inch White Rope, 656 Feet Model 795 : Agricultural Fence Accessories : Garden & Outdoor
I can usually find it quite a bit cheaper if I shop around.
I particularly like it for temporary and semi-permanent fences.
It's about six cents a foot, compared to about three cents a foot for high-tensile galvanized steel. But it's a lot easier to work with. It's lightweight. Cut it with scissors, tie it together with knots. The only tools you need for a day of fencing are a post pounder and a pair of scissors.
At the ends, you can use regular quarter-inch rope as an insulator and tie it to just about anything -- a tree, a post, a barrel full of rocks.
For line posts I really like a t-post with a Lockjawz insulator:
LockJawz - The Last T-Post Insulator You Will Ever Need
They stay where you put them. The only thing better for staying put is a wooden post with a nailed-on insulator but that's a lot more trouble. The catalogs are full of plastic wands and cheap t-post insulators, they're nothing but grief in my experience.
I find the fence is the most trouble-free if the rope is kept pretty tight. With a tight rope the line posts can be pretty far apart, depending on the terrain I'll sometimes only have posts at peaks and valleys. Just for fun one day I hooked up a scale to a rope I was tensioning the way I like it, the scale read 100 to 120 pounds of tension. To tension the rope, I use a trucker's hitch (
Trucker's Hitch Knot | Knot Tying | Survival Knots). But I put two plastic donut insulators in it. They act as pulleys and reduce the friction, making it easier to get it tight, and also reducing wear on the rope. At whatever I'm attaching to I put a piece of rope, and run it through the center of one donut. The electric rope goes around that donut and it will spin like a pulley. The other donut has the electric rope go through it, and it sits inside the loop of the trucker hitch. So when you tighten the trucker hitch it's spinning on the two donuts. If that explanation doesn't make sense I can try to do better.
This is the donut insulator I use:
Plastic Donut Insulator Black
I can usually find them on Ebay in bulk much cheaper though.