[Tom Line]

Spectra is one option. It absolutely does not stretch. It's a polyethelene (plastic) based rope / line that many sailboat racers use in place of aradmids like kevlar due to it's high UV resistance and (comparatively) long life. It weighs next to nothing and is far stronger than steel rope. However, it does chafe and will wear through if it crosses sharp corners. It's also pricey compared to wire rope, but you can use tiny pulleys (less than 1 inch diameter).

Of course, the good old standby of cable works well too. Heavy, must have larger pulleys, but has excellent indicators of failure when strands begin to part.

Then there's chain.

You can always use chain to power the first stage, and spectra or wire for the others. That's what we did in '08.