In reply to SteveM: As others have said the Edelrid 19G are the absolute lightest. However they are EXPENSIVE, especially the quickdraws. They would be amazing where weight is absolutely critical but a full rack of them might not be ideal on grounds of both cost and handling.
Having a few (perhaps 4?) 18cm 19G quickdraws for serious routes where you want a really big rack with twelve to sixteen draws would be really helpful and might be a decent compromise.
However in terms of value for money and general performance I think the Ocun Kestrel at 25g in weight and £5 per krab is pretty unbeatable. I'm very likely to swap the racking krabs on some of my cams from Spectre 2s to Kestrels.
I've currently got some of the 19Gs and Kestrels as well as lots of DMM Phantoms and Spectre 2s, plus numerous WC Helium quickdraws.