Tony Stark is always years ahead of the top inventors in his field.
The rock-it launcher (Fallout 3) is made of spare parts, and launches random junk as ammo.
An example of a weapon made by use of 2x4 technology.
Link (The Legend of Zelda) often understands how advanced technological devices work, no matter how foreign they may seem to him.
Spider-Man (Marvel) is incredibly intelligent and has designed most of his own equipment and weapons, most notably his web-shooters and spider-tracers.
River Tam (Firefly/Serenity') is well-known as a child prodigy with extensive mechanical knowledge, among other talents.
Miles "Tails" Prower (Sonic the Hedgehog) is a mechanical prodigy.
Doctor Eggman (Sonic the Hedgehog)
Rotor the Walrus (Archie's Sonic the Hedgehog)
Manic the Hedgehog (Sonic Underground) has incredible skill in mechanics and computers.
Donatello (Teenage Mutant Ninja Turtles)
In addition to her magical powers, Willow Rosenberg (Buffy the Vampire Slayer) has enough knowledge of computers and robotics to repair and reprogram humanoid robots.
Bulma Briefs (Dragon Ball) has such skill with mechanics that many experts and scientists are unable to understand the machines she creates.
Antonio Garcia (Power Rangers Samurai) has sufficient knowledge of mechanics to repair Zords and reverse-engineer the Samurai Rangers' technology to create his own Morpher.
Tech E. Coyote (Loonatics Unleashed)
Ash Williams (Evil Dead) is an incredible inventor, able to create a fully functional cybernetic hand from a metal gauntlet and create gunpowder from referencing its elemental makeup in a chemistry book.
Skylar Adams (Alphas) is a Quantitative Aptitudinal capable to intuitively invent devices years ahead of their time.
Otacon (Metal Gear Solid)
Kevin Levin (Ben 10) is skilled at mechanics, able to invent alien weaponry and even entire cars.
Professor E. Gadd (Super Mario)
Dexter (Dexter's Laboratory)
Stewie Griffin (Family Guy)
Sokka (Avatar: The Last Airbender) has an almost natural aptitude for mechanics and technology.
Edd (Ed, Edd, n' Eddy) can invent some useful devices from junk, despite being a kid.
Kelly (Misfits) displayed this at a advanced level, as shown being able to create rocket designs.
Morty Boomer (Super Smash Adventures)
Ratchet (Ratchet & Clank), like all other Lombaxes, is a natural with gadgets and technology, even building his own spaceship from scratch.
Kaolla Su (Love Hina) is a mechanical prodigy.
Bently (Sly Cooper) is extremely skilled with technology, upgrading his wheelchair with various weapons and gadgets.
Because of Sir Raleigh's evil and mad machines, he was able to build a storm machine and an entire artificial island to loot ships of their valuables.
Arpeggio (Sly Cooper) studies led him to the works of the Italian Renaissance masters, whose notes and machinery designs served as a spring board for the sinister young genius. Eventually, he constructed a large blimp, which served as his home and base of operations.
Penelope (Sly Cooper) has multiple fields in technology, including piloting, using remote-controlled devices, and machinery.
Dr. M (Sly Cooper) is highly skilled in creating powerful machines.
Tinker Bell (Disney Fairies), like all Tinker-talent fairies, possess a natural mechanical intuition.
Torbjörn (Overwatch) is a master mechanic and inventor.
Mechanicles (Aladdin (TV series))
Taotie (Kung Fu Panda: Legends of Awesomeness)