Words that are rarely used in the singular:
trivia (singular: trivium)
minutiae (minutia)
paparazzi (paparazzo)
grafitti (grafitto)
assizes (assize)
scampi (scampo)
auspices (auspice)
scruples (scruple)
timpani (timpano)
measles (measle)
Words that are rarely used in the positive:
(in)advertent
(in)corrigible
(im)maculate
(im)placable
(in)clement
(in)effable
(dis)consolate
(un)wieldy
(in)delible
(in)nocuous
(un)furl
(im)pervious
(in)sipid
(un)expurgated
(un)speakable
(im)peccable
(un)kempt
(in)evitable