Eucalyptus saxitilis - Suggan-Buggan Mallee, Mount Wheeler Mallee, Suggan Buggan Gum (southeast Australia) This rare, very hardy species is similar to E. glaucescens but does not grow as large.  The glaucous juvenile leaves and the adult leaves are similar to those of E. glaucescens, but the tree is more likely to grow as a multi-trunked mallee.  It has a restricted distribution in Australia and remains extremely rare in cultivation, but would probably be hardy to at least 0 to +5°F.

Eucalyptus scoparia - Wallangarra White Gum, Willow Gum (New South Wales) This small, fast-growing euc has been popular in Australia for many years as one of the most ornamental euc species.  It has attractive willowy leaves and grows rapidly into a small tree with smooth white bark.  7 - 15°F.

Eucalyptus smithii - Gully Gum (southeast Australia) This fast-growing, medium to large tree has rough bark at the base and smooth bark on the upper trunk and branches.  The adult leaves are dark green and narrow, and the juvenile leaves resemble those of E. viminalis.  It is adaptable and will tolerate dry conditions.  Although not particularly montane, it is probably hardy to about 8 - 14°F.

Eucalyptus spectatrix - (New South Wales) An interesting mallee relative of E. fraxinoides, coming from a very restricted area on mountaintops near the coast, which differs from E. fraxinoides in having green juvenile leaves and a much smaller stature.  Not known in cultivation but would probably be hardy to 8 - 15°F.

Eucalyptus stellulata - Black Sallee, Black Sally, Muzzlewood (southeast Australia) This medium sized tree (to about 50 - 60') is one of the best eucs to use as a shade tree for its thick crown.  It has rather short adult leaves with parallel veins, and it is therefore sometimes considered a "snow gum," but interestingly the juvenile leaves are rounded, opposite and sessile.  Unlike the true snow gums, it inhabits valley floors and boggy places where the drainage is too poor for other snow gums.  This species is not always recommended for poorly drained sites in cultivation, however, since like E. regnans it may blow down.  It also has interesting olive green and white peeling bark on the upper trunk and limbs, and the tiny fruit pods preceded by attractive star-shaped flowers are intriguing.  An easy and reasonably popular landscape tree in cultivation.  -1 to + 8°F.
Photos of Eucalyptus stellulata
Eucalyptus stellulata is also featured in Milligan Seeds and Trees Gallery: (tree in habitat)

Eucalyptus stentosoma - Jilliga Ash (New South Wales) This large tree is generally similar to E. fraxinoides and probably no hardier.  Not common in cultivation.

Eucalyptus stricklandii - Yellow-flowered Blackbutt, Goldfields Yellow-flowering Gum, Strickland's Gum (Australia) Of the subtropical gums with showy flowers, another hardy-ish species that might be worth trying in southwest Britain and such places, in the same manner as E. leucoxylon.  This medium sized tree has bright yellow flowers in the summer and dark furrowed bark.  Hardy down to around 15 to 20°F or maybe a little lower in dry climates.

Eucalyptus stricta - Blue Mountains Mallee Ash (New South Wales) A small, interesting, and fairly slow-growing euc.  It usually makes a small tree of only about 15-20' high and may have single or multiple trunks.  The outstanding feature is the flowers--in fact, this species possibly has the showiest flowering of any of the hardy eucs.  The lanceolate green leaves are also attractive.  5 - 12°F.
Eucalyptus stricta is featured in Milligan Seeds and Trees Gallery: (tree in flower)

Eucalyptus strzeleckii - Strzelecki Gum (Victoria) A member of the swamp gum group, closely related to E. ovata.  It differs from E. ovata in being a taller, straight-trunked tree and having somewhat narrower leaves.  Rare in cultivation.  Probably hardy to around 10°F.

Eucalyptus sturgissiana - Ettrema Mallee (Tasmania) Another Tasmanian rarity, this sprawling mallee with beautiful blue-grey leaves looks almost something like E. macrocarpa of Western Australia, except without the showy red flowers.  The leaves are predominantly in opposite pairs, but wide at the base and tapering to a point.  Very difficult to obtain.  Hardy to 5 -10°F.
Eucalyptus sturgissiana is featured in Milligan Seeds and Trees Gallery: (leaves)

Eucalyptus subcrenulata - Alpine Yellow Gum, Tasmanian Alpine Yellow Gum (Tasmania) Another very hardy and attractive Tasmanian tree, closely related to E. johnstonii and E. vernicosa.  It has very attractive bark that peels in vertical strips revealing yellow, tan and green patches; and the leaves are also a nice very deep green.  The juvenile leaves are opposite, with square stems, and the adult leaves are pendulous.  Like E. johnstonii, it also has valuable timber; however it is quite a bit hardier and not quite such a large tree (usually about 50 - 120'), tending to keep its lower branches.  Fast growing and tolerant of a wide range of sites, including poorly drained soils.  -2 to +10°F.
Photos of Eucalyptus subcrenulata
Eucalyptus subcrenulata is also featured in Milligan Seeds and Trees Gallery: (bark of trees in habitat)