Prominent descendants: Phoenicians, Celts, Medes, Greeks and Romans.

Links open with New International Version (NIV) verses. Unless otherwise stated, links applicable in both King James & NIV.

Unfortunately, the Bible doesn't cover much about this son of Noah, aside from his birth in Genesis 5:32. Most of what we know about his descendants has been gleaned from history due to the lack of written records. The "Table of Nations" or Genesis 10 records the patriarchs of this family line as well as the lineage of mankind as a whole after Noah's family exited the Ark on Mount Ararat.

Japheth's descendants became the Phoenicians & mostly spread into Europe. Phoenicians are quite likely the ancestors of the cultures that settled on islands such as Cyprus, the Minoans on Crete and probably the Vikings as they traveled the seas looking for their own suitable homelands. This is why Genesis 10:5 states:
5. (From these the maritime peoples spread out into their territories by their clans within their nations, each with its own language.)

Very little was known of one group of descendants of Gomer called Cimmerians. The only reference I could find was in the Holman Bible Atlas showing them living just south of the Black Sea where Asia Minor meets the Middle Eastern mainland. They helped bring the demise of the Assyrians.

Etruscans seem to have been Phoenicians who settled on the Italian peninsula. Tuscany was a nod to their presence there.
In Macedonia/Greece, the Greeks emerged and thwarted an invasion by the Persians.
The Romans overcame the Etruscans in time, adopted some of their technological advances and developed use of chariots like the Egyptians.

The Celts, who were also said to have descended from Gomer, roamed Europe then clashed with the Romans. They were very fit people like the Greeks who traveled primarily on foot and horseback. In this manner, they ran parallels with the Native Americans. The Celts survived in the British Isles, especially north of Hadrian's Wall, while most of southern Europe had been incorporated into the Roman empire.

Scythians, who appear to have come from Gomer's son, Ashkenaz, also lived outside the Roman empire in the region where Russia developed as a nation.

Germanic tribes lived among the Vikings who also were probably from the line of Japheth. Collectively, the Germanics, Celts (including the Franks) & others led to the downfall of the Romans despite having some of their own people used as Roman soldiers or mercenaries.

History tells us that (Germanic) Anglo-Saxons and Celts settled Britain, the Franks established France and other European nations developed.