Ken covered the terrestrial lefties pretty thoroughly.  One additional group
that comes to mind is the Achatinella snails and related groups, from Hawaii.
What is interesting about this group (like the Amphidromus group Ken
mentioned) is that within the genus there are species that are normally
sinistral (left-handed), others that are normally dextral (right-handed), and
still others that can go either way.  Many of the Achatinella species have
known specific ratios of lefties to righties.  For example, one species may be
90% sinistral, another species 70% or 50% sinistral.
In the marine world there are also a number of normally sinistral species,
though not as many as there are among the land snails.  Two deep water whelks
of the family Buccinidae, Pyrulofusus deformis and Neptunea contraria, are
sinistral.  The genus Antiplanes (family Turridae) and the genus Sinistralia
(family Fasciolariidae) are both sinistral.  Of course, the two Florida
Busycon species, B. perversum and B. contrarium, are lefties.  I think this is
the "whelk family" Sylvia referred to.  The genus Triphora has many sinistral
species, and the species Cerithiopsis sasamorii from Japan is also a lefty.
These are all species that are considered "normally" sinistral, that is they
are almost always left-handed.  But even these can occasionally be right-
handed, and many normally dextral species can occasionally be sinistral.
There is a little deep water Turrid in west Africa (the name escapes me at the
moment) that is about 50% sinistral across the species, but in individual
populations, either the sinistrals or the dextrals may predominate.  The
family with the greatest incidence of "abnormal" sinistrality is, I think, the
margin shells (Marginellidae).  Lefties are found in many species of margin
shells, and in some species as commonly as 1 in 1,000.  Voluta vespertilio
(the bat volute) also has an unusually high incidence of sinistrality, about 1
in 10,000 specimens, if I remember correctly.  That may not sound like much,
but only a single sinistral specimen has ever been collected of Voluta
(Scaphella) junonia, out of hundreds of thousands of specimens.  And of
course, there are many species in which no sinistrals are known at all.
Paul M.