The Byzantine Empire is the overlooked civilization in European history. Byzantine scholars are the reason we know so much about ancient history, it was their culture that did so much to collect and store Roman and Greek literature after the invasions of the Western Roman Empire. They've been the subject of Western bias for the last thousand years, which is why popular opinion sees Europe from 476-1000 as the Dark Ages when in fact there was a thriving empire based in Greece which really only went into real decline after the death of Basil II in 1025

The Indus Valley Civilization. They're roughly contemporaneous with the ancient Egyptian and Mesopotamian civilizations, and had about the same level of development. However much less is known about it, and the first excavations weren't until the 1920's. They had major cities, but didn't leave many large monuments, palaces, etc. That's led to a theory that it may have been less hierarchical and more egalitarian than Egypt or Mesopotamia.

Many nomadic and semi-nomadic peoples are easily overlooked. My personal 'favorite' are the Galatians, an ancient confederation of Celtic tribes that migrated to/invaded Anatolia. They were originally from what is now southern France and were a part of a larger eastward migration wave of Celtic peoples. The Galatians would either be at war with their Greek neighbors or serving in their armies as mercenaries.

You could look into my area of West Mexico with the shaft tomb culture and later Teuchitlan Tradition.

The Hopewell and Mississippian cultures in North America are pretty interesting in terms of trade and cultural influence, so you may want to look into those. Sorry I can't provide sources to get you started or better formatting, I'm on my phone.

Cant leave out the vikings, they are a pretty fascinating culture of people. A war dominated culture that had a brutal and barbaric set of morals and values. There's a pretty historically accurate show on the history channel called vikings which loosely follows the life of a famous viking named ragnar lothbrok. It gives insight into their faith and lifestyle. They thrived from about the 8th to 11th century.

The Srivijaya Empire which was based on the island now known as Sumatra in Southeast Asia. They are known for their naval power, seafaring capabilities, and trade.

Also in the region are the Khmer, which were based in present day Cambodia. They built the well known temple Angkor Wat.

There are also lots of African empires and civilizations that get less attention than European or Asian groups.

Sorry, but this is a poll or "throughout history" type question, which is against the rules of the sub. As much as I'm all for learning about lesser-known cultures, I'll have to remove it. If you manage to find some basic info on lesser known civilizations and want to ask a specific question about them, feel free to make a new post.