Animal husbandry and plant breeding. Great side classes for monster tamers and herbalists/alchemists, among others.
Tailor and leatherworker. They aren't uncommon, but people seem to underestimate the value of strong, well made clothing. In some cases, the bonuses would be better than what an Armorer could manage. Especially worth noting is that armor without clothing is extremely uncomfortable. If that armor is Fucking Indestructible Dorf-Made Dragonscale Armor™, then it will probably chafe really bad. You'd need to pad it with something equally durable.
Pottery and glassmaking are quite rare, but perhaps not as useful as other skills. They definitely qualify, but usually they get overshadowed by earth magic or barrier magic.
Working with bone and other animal components other than leather or scale often gets overlooked, or just straight up integrated into metalworking for convenience.
Jewelry is often utilized in RPGs, and so it's actually quite common in games. I don't want to say it's rare in books, but it's sometimes overlooked.
Mechanical skills are somewhat underutilized, but probably more because fantasy authors are obsessed with medieval stories for some reason. I'm including detailed creations like tools and gadgets, as well as actual machine crafting like engines, firearms, and watches. It's also easier to justify since detail work like this is hard... but if you get a skill for it like all the other skills/classes, then that kinda goes out the window.