In reply to idiotproof (Buxton MC): The RP Brass Nuts are probably the most famous ones. They're the ones that started it and many people consider them as the best ones. They're very hard to get though. DMM has "kind of similar" nuts made of brass called offsets + couple of other nut types that fit into "micro" -category (i.e. smallest pieces fit <5mm cracks). This being said many others have have such nuts too, including for instance Black Diamond, Wild Country, Zero G, Trango, Omega Pacific, Climbing Technology etc. ...CAMP ball nuts are also pretty interesting, kind of "active" nut, they should be the smallest pieces of active protection in the world.
Here's a link where from you can find the specifications + some ratings of such nuts that fit to micro nut category (12 different):
http://www.tribevine.com/products?open=true&ids=6220,6574,6823,6575,622...