Passive protection is simply gear that wedges itself into a crack. It doesn't move or do anything exciting, it just sits there. But it is the most common, and possibly most useful piece of gear. A good placement can be bomb proof. But as usual the smaller the size, the less contact it has with the rock, so the less holding power.

Nuts were originally smooth pebbles that had been gathered from a beach or where ever. These were wedged into cracks and threaded or wedged tight with knotted a sling. A steel krab would have been used to clip the rope to the sling. From this, simple machine nuts that had their thread removed were used. The thread was removed to reduce wear on the cord they were threaded on. These were to say the least, c**p. The first commercially designed nut was the MOAC, which was simply a wedge tied onto some rope. Originally designed by John Brailsford. Now nuts tend to be threaded onto wire, which allows more precise placement and removal. The other difference between modern and the Moac is that todays nuts often have curved sides.

The other developments were hexagonal nuts, probably devised from the original machined nuts. These hex's were offset so they would twist and expand into the cracks.

Wild Country Rocks

These are a fairly close descendent to the MOAC. The only modifications are the concave sides, allowing a little amount of camming. All sides are tapered, to allow multiple orientations. Also they are threaded onto wire, which allows them to be placed a lot easier. But it does increase the movement transferred from the rope to the nut. So it has to be extended. It used to be possible to get them unthreaded, so you could put them on cord. This was only possible on the larger sizes.  
 
DMM Wallnuts

Very similar to the rocks. The main difference being that large chunks on either face have been removed, allowing it to catch on nodules in cracks, and give a little better protection. The wire is concealed on the nut, protecting it from damage. The sizing are slightly different from rocks so they make a good half set. The most useful size seems to be 3, 4, and 5. Also due to the removal of non-essential metal the set is slightly lighter than a set of rocks.
     
HB Brass Offsets

These are the smallest nuts I own. They are extremely tough brass welded to the wire. All sided are offset and tapered, allowing a very diverse range of possible placements. I wouldn't like to take a large fall on these, mostly due to the fact that the wire has quite a small diameter. But they are a great mental help when the wall looks blank.
     
Camp Hexcentrics

Although not strictly a passive piece of gear, they can act in a passive way. They are basically a hollow, hexagonal nut, that is threaded with rope. The hexagonal profile has all sides offset. So when the rope pulls, it twists the nut, pushing the sides into the rock. You can get bomb-proof placements in parallel sided cracks. The ends are tapered so they can act as a nut if needed, which I find very useful. The smaller sizes have such a small size of expansion, and the rope needed to thread them is of such a small diameter, that they are not really practical. But the larger sizes have lots of uses, not least to get sea gulls away from you on sea cliffs. The picture is of the Black Diamond Hex on wire.
      
Wild Country Rockentrics

I actually designed this a couple of months before it was released, as part of my design-technology A-level. I came to the conclusion that it wasn't much, if any, of an improvement over what is currently available (I can provide documentation). But having used them I do see that there is a need for them. They work best as purely passive nuts in large cracks, they work better than a hex. They go much larger than wallnuts and rocks. Different from hexes they use spectra, and are thus pre threaded.    
 
Kouba Nuts - Czech

Kouba do a good selection of wires at about a pound each. Size 4 shows 10KN through to size 1 at 6 KN. Available from most climbing shops in the Czech Republic for which there is a couple in Prague. These wires are made from brass which makes it more difficult to get them to bite into the rock. The brass doesn't deform like aluminium can, which, combined with their stubby appearance makes them very difficult to place well. But for less than a fifth of the price of rocks or wallnuts they are a very tempting buy.