Artificer

Artificer is the most common term used in fantasy games for a character archetype or class focused on the creation and use of gadgets and devices. Whether a pulp gadgeteer, a mad scientist, or even outright called an artficer, they're somebody defined by the way in which they can either use artifacts to fight or else their skill at making stuff.

Needless to say, artificers mostly tend to show up in steampunk, dieselpunk or magitek settings.

Dungeons & Dragons

This archetype actually goes back a long way in Dungeons & Dragons, all the way to the Skygnomes of Mystara. The most "classical" D&D example, unfortunately, are the incredibly irritating Tinker Gnomes of Dragonlance - gnomes cursed by Reorx to constantly experiment with science, but never to actually achieve anything.

The archetype came into its own with the Artificer class in Eberron: as Eberron was built to explore the logical conclusions of D&Disms, magical engineers were a natural result, and this gave rise to the Artificer class. It was more or less an arcane buffer, using a complex ruleset called "Infusions" to temporarily enchant items and augment constructs, as well as being better at making magical items and creating construct sidekicks.

In Dungeons & Dragons 4th edition, it came into its own as the Arcane Leader; embracing its status as the Wizardly equivalent to the Cleric, it specialized in healing spells and temporarily buffing armor & weapons, from giving your guys flaming swords to dispatch trolls to making your armor electrocute anybody trying to stab your ass.

In Dungeons & Dragons 5th edition, initially, the Artificer was released as a new subclass for the Wizard in the "Eberron Update" official booklet. It was a source of much skub, not only from the perceived downgrade, but because it simply wasn't very good. Then, to everybody's amazement, it got re-released as a new Int-based 1/3rd caster class in the January 9 2017 issue of Unearthed Arcane. In this case, it also doubles as an Alchemist or a Gunslinger.

Deadlands

In Deadlands Classic, the Mad Scientist Arcane Background is essentially a magitek artificer, using inspiration secretly provided by demons and super-fuel compressed from damned souls to engineer all manner of impossible devices. In Hell on Earth, you got Junkers, who are essentially a post-apocalyptic counterpart to the Mad Scientists. And in Lost Colony, the Mutes use arcane-fueled mentally-controlled nanobot swarms to achieve the same result.

Ars Magica

The guild of Hermetic magicians known as House Verditius in Ars Magica doesn't use formulaic spells like the rest of the gang; instead, they craft items that will express their spells, and these items can even be used by non-wizards, although they do not last long enough to make true wizards obsolete. Sometimes the rules of the game describe House Verditius as alchemists, expressing their spells with potions, vapours and ointments instead. Their heraldic sign is a hand with a ring on each finger.

Mage: The Ascension

In Mage: The Ascension, this is essentially the archetype tapped by the Sons of Ether and the entirety of the Technocracy, all of whom channel magick through pseudo-technological props and mediums.

Warhammer 40K

Besides using "artificer" to mean "a truly skilled craftsman" (artificer gear is very valuable and much more effective than normal), the Ork Mekboyz are a classic example, as their technological creations are mostly held together by the psionic energies of other orks.