Some of the material in Barbarians first made an appearance in my campaign. Some had been tried out in a private game run by Christopher Rice, but this is about what first showed up in my games, often with the names filed off. Other are homages to things in my game.
The Shirtless Savage - Bjorn was the first in my own games. It was just good timing - Vic had contacted us a long time ago about playing, but inertia being what it was, it took us a long time to invite him for a tryout. Based on the characters he was debating playing, I offered him a chance to run something not yet in print and playtest it. He jumped on it, and his in-play demonstrations of the template led me to make some last-minute changes to make it a better one.
Savage Warrior - Ditto. Vic ran the first one in my game.
Weapon Master (Barbaric Weapons) - Hasn't shown up in my game, but it was originally written for Honus Honusson, who wanted some way to become a bit more combat-centric. So I kept tinkering with a long-term goal WM variation for his barbarian.
Raggi's Reaver - this is the axe Raggi picked up from the spectre. Actually, that treasure and monster is a self-homage to my 1990s GURPS 1st edition game, and the axe in question was used by Skullcrusher Kurgan the half-ogre, and later by Thylack the barbarian. It was even loaned to Vryce's player's half-ogre in a side game we ran in the same game world. So putting that axe into the book, with Raggi's personality embedded into it (to be fair, Skullcrusher thought All Out Attack was the only move worth doing and Thylack was a berserker, too), was a no-brainer.
Ferocious Beard - Two words: Jon Blackbart.
(Would have been good for DFD: Dwarves, too, but I'm not planning on writing that one.)
Two-Handed Axe/Mace Damage - We've been doing some of this since before the beta for GURPS Low-Tech. If it seemed like the axe-wielding maniacs did suspiciously high damage, this was why.
The customization notes for Armored Barbarian, Flailsman, and Shieldrusher are all based on Honus Honusson (seen here in solo action), who made shield rushing into a core combat feature in my games.
There are one or two other items which might have made an appearance, but the players went left when the thing in question was off to the right. Or just haven't reached it yet due to scheduling of play. It happens. But now you might have some idea what you saw but didn't realize you were seeing, or why something in DFD1 seems so familiar . . .