Wolf and Tiger Nomads: The Cold North of the Flanaess is practically all "barbarian" cultures. The wolf and tiger follow totems of their appropriate animals and are plains dwelling people of mixed-race, but mostly Baklunish origin. The tiger nomads come into conflict with their cousins and the "civilized states" of the Baklunish more often, whereas the Wolf Nomads also have Iuz as a neighbor and have warred with the demigod's empire a few times in history. They also abutt to another barbarian culture...
Rovers of the Barrens: These Flannae horsemen are to the east of the Wolf Nomads and are the traditional hunter gatherers you'd expect in the cold climates. In some sources they've even been equated to American Indians, though its not a required analogue. These nomads too run afoul of the evil Iuz and are also hated by the Stonefists.
Hold of Stonefist: This far north pocket of barbarians have a couple towns but they are more savage and brutal than most any barbarian culture in the north. Their 'battle rite of fitness' is unique to the setting and shows how it is survival of the fittest in their land. Needless to say the Stonefists are not loved by anyone. They are featured as bad guys in the module Howl From the North.
Ice, Snow and Frost Barbarians: When someone mentions barbarians, these three mountainous countries are the first thing that comes to peoples minds. They are the viking analogue and deservedly so. They have their own rich histories and adventures, also featuring in the Five Shall Be One/Howl From the North epic plus other mentions throughout Greyhawk publications. These barbarians as you would expect like to raid by longship and harass the Aerdy on the Solnor Ocean or explore the Icy Sea and beyond the oceans to the east.
Plains of the Paynims/Dry Steppes/Ull: In the Baklunish West are many many more nomadic cultures. These Mongol/Persian themed groups wander a very wide band of the maps from the Sea of Dust to the southern edges of the civilized Baklunish countries to the west beyond the Flanaess where once the Golden Horde raided eastward and went through Ket to attack the rest of the Flanaess. The Dry Steppes and Ull are very remote, sparsely populated areas with their own lost legends, many which you can find here on Greyhawkery.
Bright Desert: Even in the middle of the Flanaess within this small hill-ringed desert you can find barbaric nomads of Suel and Flan descent. The place was once the flan kingdom of Sulm but now the ruins and remnant people of this culture are ruled by the archmage Rary as seen in the sourcebook Rary the Traitor.
Amedio and Hepmonaland: The two jungle areas to the south are separated by expanses of water but linked by islands, all of which are inhabited by decidedly "barbaric" peoples of the Olman and Suel subraces (we won't go into the Touv of Hepmonaland yet). The Olman, based on mesoamerican culture, were like many races once quite civilized, but over time fell and are now hunter-gatherers competing to survive with interlopers like the Suel, who also once were mighty but fell, some say into savage cannibalism.
Are there more barbarian cultures? Yes I am sure I can spot others. The Land of Black Ice/Burneal Forest tribes. The Crystalmist Mountain snow elves and maybe even the humanoid tribes of places like the Pomarj could count too. That's a debate for another day though.