I started a thread under the old Cubits before ATP got going. Lynn may know where to find it here. She also has a few that are very tiny as well, one an arachnoidum whose name escapes me at the moment.
Some of the smaller semps (typically about 1/2" in a colony) that I grow are listed below. I also have read about an arach that only gets to about 1/4", but sadly that one seems to be lost from the hobby (at least at the moment). I do not have a name for it and doubt that it would remain that small unless grown in a colony.
S. barbulatum f/'Valle Quazarro'
'Valle Quarazzo' (hope I spelled it right, always seem to misspell this one) is interesting in that it seems to go through population cycles more drastically than other semps. One year I'll get an explosion of offsets, other years most will die off. Very prolific, but hard to keep looking nice in a container.
S. X barbulatum 'Hookeri', S. barbulatum X Hookeri
There may be two versions of the 'Hookeri" floating around, as I have two under slightly different names (and from different sources) that look a bit different, even though grow in the "same" area of my collection separated by only a foot or two.
S. arach. 'Linde' (in a colony, stays about 3/4")
'Linde' is one of the older members of my collection, going back probably about 15 years for me and my first arachnoideum. I have always grown it in a container that keeps the rosettes small. I like how the webbing gets bunched up to make circles around the rosette. This does not happen when grown with lots of available space.
S. 'Speciosum' (3/4 - 1" in colony, makes a great clump)
'Speciosum' makes beautiful clumps. Very well behaved so far. I've only had this one a few years.
The smaller ones need more protection from the sun and more water than the typical semp. I'm always on the lookout for more, even though they are slightly more of a challenge. Check out SMG Succulents species page for more of the tiny ones. Edelweiss Perennials also had a few, as I recall.
Monanthes polyphylla
Lastly, and slightly out of place, is one of my non-semp favorites. It is from the Canary Islands, is not hardy (frost will kill it very quickly), does not tolerate much water but is very small. It was miss-ID'd when I first saw it as Monanthes brachycarda(spelling?), but is actually Monanthes polyphylla. It is a delight. I never water it, just mist it. It is also very fragile and falls apart when handled. I believe it is a relative of the Semp/Sedum families, but not sure. Killed mine off last year going into winter but found some more this spring and its doing well so far, including even a successful transplant (lost about 25% due to the handling, however)
