Weekend Inventor

In fiction land, inventions are mind-boggling easy to make. This fellow is not a Bungling Inventor, The Professor or a Teen Genius. He might have a degree in Art History. However, he has managed to build some item of technology the entire industry could not replicate, probably in his garage using the knowledge he got from flipping burgers at McDonald's. Mostly he uses it for some mundane reason like fighting crime or making breakfast, or he just has it lying around so it can be given to the main character.

Comic Books

 * Peter Parker invented a fluid that can be used as a stretchy rope and that hardens into a unbreakable barrier, in high school. There's been various explanations for this over the years. Some versions just have him as a budding super-genius on the same level as Tony Stark or Reed Richards, but some authors ignore this because it clashes with his "everyman" portrayal. Once divisive explanation is that he gained instinctive knowledge of the composition of web fluid as part of his spider powers. Some continuities forgo this trope altogether and say that his father invented it.
 * The original Superman claimed to have made his suit himself from a fabric he invented. You know, the suit that stays on him when he flies through the sun. Imagine the commercial and military applications for that.
 * From time to time Superman's powers include super-intelligence. It's usually ignored because making him the most powerful and smartest person on Earth makes it really hard to write stories about him.
 * In one episode of The Adventures of Superman, Jimmy Olsen invented an Anti-Gravity solution. In another, Perry White discovers a way to extract Uranium from seawater.
 * In Ex Machina, Mitchell Hundred got a full blueprint for a jetpack from the aether in a dream, presumably as a side effect from his machine-talking powers. He also built a machine to nullify his own powers, some laser guns, taser gloves, a multi-sensor helmet, and maybe some other cool stuff we haven't seen yet. Justified, since his superpower involves communicating with technology.

Literature

 * Norman the shopkeeper from the Brentford trilogy. His illuminated cowboy costume was less successful, as a static build-up caused it to collect sawdust and it (predictably) suffered a castastrophic short when someone tried washing it off with a soda siphon.

Live Action Television

 * On The Big Bang Theory, when he lost his job, Sheldon, a theoretical physicist, somehow managed to genetically engineer glow-in-the-dark goldfish in under a month.
 * Actually, that's not that difficult in practice. It's just that you'd need thousands of failed recombinants, and far longer than one week to let them mature, but still completely doable.

Web Comics

 * Nick of General Protection Fault has the Inventor's Gene, justifying the trope, as does his uncle, Otto Wisebottom.

Western Animation

 * In the first episode of The Flintstones, Barney invents a flying machine. In later episodes, Fred manages to create various concoctions out of his garage, including a shrinking potion, a super glue and an invisibility formula. And all this using Stone Age technology!
 * Maybe they're ante-Deluvian cavemen.
 * Mr. Turner on The Fairly Odd Parents makes several homade (and dangerous) items in his garage.
 * Huey from The Boondocks.

Real Life

 * A significant proportion of the code in most Open Source software is written by self-taught hobbyists working in their spare time.