http://www.jungleproducts.co.uk/ks/exaform-speed-up
I have recently got one of these. I’m chuffed to bits with it. Wanted a simple, no frills, unlikely to fail, system of having post up or down but better than a quick release.
Worth noting – it’s designed to work properly when the seatpost shaft is clamped in the bike. When I tried it in the shop (still in the packaging) I was a bit dubious of its quality. However, put it on the bike and give the stanchion a spray with silicone seal and it’s great. Post can stop at any part i.e. not just fully up or fully down. Exactly what I was looking for. Highly recommended so far.
Not sure it’d be good for trail centre riding (constantly having post up or down) but for riding in the hills when it’s either up for the grind up the peak or along the flat or slammed down for the descent it’s perfect. And it’s relatively light.