Power-ups are to the Super Mario franchise like items are to The Legend of Zelda: intangible and plentiful. So when one Super Mario fan set out to create pixel art of every power-up in the franchise, they were in for a lot of work as there are dozens to depict.
Back in the original Super Mario Bros., the power-ups at the Italian plumber's disposal were a bit more streamlined. Players could get a Super Mushroom to grow in size and take an extra hit, a Fire Flower to both grow and shoot fireballs, or the Star to become invincible for a short period of time. As the series would progress though, and especially from 2D to 3D, with every new title it seemed like there were new power-ups being introduced. Many of these power-ups are uncommon in the greater Super Mario series like the Gold Block from New Super Mario Bros. 2, that gave Mario coins as he sprinted.
Reddit user BausOfBacon has painstakingly created pixel art of every Super Mario power-up in the style of Super Mario Bros. 1. There are 52 sprites in total (51 when not counting the blank space for Super Mario 64's Vanish Cap), all of which fit inside the 32 by 32 pixel count used in the original Super Mario Bros. The power-ups of Super Mario Galaxy are very prominent in BausOfBacon's work, those being Spring Mario with his coil body, Bee Mario whose overalls alternate brown and yellow, the self-explanatory Boo Mario, and the entirely light blue Ice Mario.
BausOfBacon speaks a bit about the work that went into this project in the comments of their post. According to them, there were a handful of really easy power-ups to make, them being the Super/Rainbow Star, the Ice Flower, and the Super Bell as were "just a couple of easy palette swaps."
On the other end of the spectrum, the hardest power-ups were the Master Sword and Cloud Flower, the former transforming Mario into Link from The Legend of Zelda, and the latter giving Mario a large cloud hat. Rather than simply using the Link sprite that was used in Super Mario Maker 2, BausOfBacon made a unique design more akin to Super Mushroom Mario.
Some players may be wondering where power-ups like the Coin Box and Cannon Box from Super Mario 3D World are, and based on BausOfBacon's work, they may just not consider them as power-ups like the rest. In the games they are present in, they can be used in addition to a traditional power-up like the Fire Flower or Super Bell which in turn, detracts from the Coin Box and Cannon Box uniqueness. Integrating those power-ups here would mean for a lot of Super Mario sprites to be rendered.