Depending on how common, "/usr/share/dict/american-english", suggests

honorificabilitudinitatibus
unimaginatively

But my "/usr/share/dict/american-english-huge" gives

aluminosilicate
aluminosilicates
cytomegalovirus
cytomegaloviruses
generalizability
hexosaminidases
honorificabilitudinitatibus
honorificabilitudinity
hypocotyledonary
ineradicability
overimaginative
paramyxoviruses
pararosanilines
parasitological
polyvinylidenes
supererogatorily
tenosynovitises
unimaginatively
verisimilitudes

And if you want to get crazy, the "american-english-insane" yields

acetonylacetone
acetylacetonates
acetylasalicylic
adenolipomatosis
aluminosilicate
aluminosilicates
anatomicomedical
categorematical
cycadofilicales
cytomegalovirus
cytomegaloviruses
epicoracohumeral
epicotyledonary
gelatinizability
generalizability
hamamelidoxylon
hemadynamometer
hematocytogenesis
hematocytometer
hematodynamometer
hemocytogenesis
hepaticological
hepatomelanosis
hepatoperitonitis
heterometabolic
hexosaminidases
honorificabilitudinitatibus
honorificabilitudinity
hydatopyrogenic
hyperanakinesis
hyperoxygenized
hypocotyledonary
hypovitaminosis
ineradicability
katakinetomeric
laparohepatotomy
manipulatability
medicobotanical
metabolizability
monocotyledones
nonapologetical
overeducatively
overelaborately
overimaginative
overimaginatively
overimitatively
papulovesicular
paramyxoviruses
pararosanilines
parasitological
pericanalicular
perihepatitises
polycotyledonary
polyvinylidenes
reticulatoramose
reticulatovenose
reticulocytosis
retinopapilitis
semicaricatural
semifiguratively
semimineralized
semipopularized
semipyramidical
siderosilicosis
superacidulated
superelaborately
supereligibility
supererogatorily
superoratorical
superoxygenated
tenosynovitises
tenovaginitises
tocodynamometer
tokodynamometer
unimaginability
unimaginatively
unimolecularity
unoriginatively
verisimilitudes
vesiculotubular

Hurray for regular expressions

Gumnos,
as Anu asked the question with today's word, you should send him your answer[s], as he may not see it here in this user based forum.

Wow Gumnos! some great words there.....how long were you working on those?

Sorry, just getting back to this (apparently the "email me with follow-ups" checkbox didn't send me any of your replies). It was a simple regular-expression run against a local dictionary on my computer:

grep '^[aeiouy]\?\([bcdfghjklmnpqrstvwxz][aeiouy]\)*[bcdfghjklmnpqrstvwxz]\?$' /usr/share/dict/words | grep ...............

which roughly translates to "allowing one optional vowel at the beginning of the word, find any number of consonant-followed-by-vowel pairs, optionally followed by one consonant at the end; search the local dictionary file for anything that matches the pattern; take the results and filter them for matches containing at least 15 characters in them". There's something to be said for geekery.