I picked one of these up at Wally world, for about $12..
It's a 'Bell' key lock/kevlar cable combination (the kevlar wrapped cable's supposed to be 3 times harder to cut than cable alone)..
The neat thing about it, is it comes with a bracket that attaches to your seat tube, and the lock body has a cutout that fits in it, and a spring loaded catch that keeps it in place.. Best lock system I've used so far.. Everything's covered in nylon or plastic tubing so nothing to scratch.. And you don't even notice it's there until you need it. I use a piece of cord tied around the cable to keep it in place. The whole lock/bracket combination weighs virtually nothing (so lite I haven't bothered to weigh it).
Like any lock, it's a compromise. Given enough time somebody can get through anything.. But this absolutely works great. And the attachment body is made of nylon and doesn't weigh jack, so when you don't need the lock, it can stay at home and you don't notice anything's there.