From Sonic Retro

Many objects have been featured throughout the Sonic the Hedgehog series with different purposes. Some items, like Shields and Power Sneakers, are power-ups that can enhance a character's abilities either temporarily or permanently; some, like Emblems and Red Star Rings, are collectibles that are either used for a purpose or are simply for game completion; and some are unobtainable plot devices, and/or will automatically stay with the player character after they're gained.

Whether they're permanent assets or temporarily held, they're all gathered here.