I like also the playsail for its simplicity: only a rectangle (a tarp with eyelets for example) with a tether in each corner. See also The most basic airborne wind energy system .
http://2e5.com/kite/npwc/falken/ : " To appreciate how big a leap this was, consider that the state of the art briddleless single skin design at the time was something called a playsail, an ‘almost kite’ that has more drag than lift and can only be flown with a wide separation of lines."
The playsail is not as efficient as the single skin kite mentioned by Tallak, as the elevation angle is lower.