Yep, that's a game where you shoot Bowser with a Doom-style shotgun, in a Virtual Boy, the system that was so awkward it gave people migraines. When these guys say they're only doing this because they love it, they aren't exaggerating.
These guys have been around for a while, but Animal recently interviewed the people behind two companies making homebrew retro games, Good Deal Games and Piko Interactive. Both companies focus on making and publishing new games for discontinued consoles, most of which haven't seen an official new release in decades.
Some of these are relatively easy to get access to, like Super Noahs Ark 3D, for the SNES. Piko focuses on NES, SNES, Sega, and Gameboys.
Some of the other passion projects are somewhat more... obscure:
The Vectrex is a unique system; released in 1982, it has its own built-in vector monitor, like a miniature arcade machine. According to Thomasson, it's a rare find today because when the game industry crashed in the '80s, a hospital chain bought them all up and converted them into EKG machines. He talks about it wistfully, like another dude might discuss the car he had in high school. So far he's published one game for the system, and he's sold about 70 copies of it.
There are plenty of game developers who use a retro art style, and some design games that aim to be simple enough that they could have been made on retro consoles. Even going so far as to port them to a Commodore 64. That isn't enough for these homebrew developers, they make games start-to-finish designed for the NES, Atari, or Vectrex.
However, the homebrew retro market isn't very lucrative, so there's at least some thought of porting some of these games forward to more modern platforms:
Galindo wants to take Piko digital as well. So far, the fledgling company has only released physical products on cartridges, but he hopes to expand to digital distribution as soon as this year. "Once you have done a game for SNES, you can easily emulate it on all these new platforms, and release there as well," Galindo said. He didn't mention any specific platforms, but iOS, Android and Steam are all likely contenders.
There's a whole lot more info in the interview, so go give it a read!