Hellfire Saga is a full-fledged Sonic the Hedgehog 3 & Knuckles ROM Hack developed by Red Miso Studios, where Sonic goes through levels that are heavily based on classic 80/90s games and with a dark/spooky theme (Castlevania, Ghouls n' Goblins, etc).
A hack which combines Sonic 1 levels, Sonic 2 (final and prototype) levels, Sonic 3 levels, and Sonic and Knuckles levels into one game.
Mega Man X in Sonic The Hedgehog 2 is a ROMhack/mod of Sonic The Hedgehog 2 which lets the player control Mega Man X instead of Sonic, with all of his signature attacks, like the charge beam, the wall jump and the dash, and X can also get the weapons from his enemies, like in his own games.
Sonic 1: South Island Definitive is a hack by RadiantNexus Team, which seeks to reimagine Sonic 1 by rearranging levels. remixing music, adding features from future games, and readding scrapped ideas in new and interesting ways. Major features: 3 acts of reimagined level design, including one act that has yet to be fully reimagined in any other hack. A miniboss. Massive art overhaul all over the place. Music made by ProjectFM and MechaKnuckles. Menus! A custom splash screen. There's been massive engine overhauls as well, including the Sonic 3 level layout format for massive levels, and the Sonic 3 object manager to go alongside this.
Sonic the Hackable is a ROM hack of the original Sonic the Hedgehog for the Sega Genesis. Originally started as a practice project focused on level design, it has since evolved into a comprehensive "demake" of Sonic the Hedgehog 4: Episode I. The hack reimagines the modern Sonic 4 aesthetics and level concepts within the technical limitations of the 16-bit engine, featuring signature moves like the Spin Dash and Air Dash alongside entirely new level layouts and remixed soundtracks.
Knuckles' Emerald Hunt is a modification of Sonic the Hedgehog 2 that split off from another hack Sonic 2 Adventure Edition. It's main goal is to mimic the Emerald Hunting Gameplay from Sonic Adventure 2/Battle. Playable characters being Knuckles and Tikal, through each stage you must find and collect 3 Emeralds or Emerald pieces. The Game features the radar from SA2, and a secret Sandwich mode in which all the emeralds are replaced with Sub sandwiches and the Master emerald is replaced with a Subway footlong. It also features the Scrapped Hidden Palace Zone and some Sonic 1 Zones
Sonic 2: Pink Edition is a modification of Sonic 2 that replaces Sonic & Tails with Amy and Cream as playable characters, Amy's sprite being based off her modern design. The changes aren't just to the asthetics though, Amy and cream bring their own movesets to the table! Amy bringing her trusty Piko Piko Hammer to swing,jump,whirl and dash through Robotnik's obstacles. Cream bringing the spindash and ability to fly, oh! don't forget about her trusty Chao, Cheese who can attack enemies, even bosses, from a distance!
Metal Sonic in Sonic the Hedgehog 2 swaps Sonic out for Metal Sonic from Sonic CD, you can choose between the Original moveset, Sonic CD, or the 2011 remaster moveset from Christian Whitehead's port of Sonic CD Metal will once show Robotnik he's not a bot to be forgotten...
Amy in Sonic 1 is a modification of Sonic 1 that swaps sonic out with Amy Rose from Sonic CD, bringing with her, her move-set from Sonic Advance. Using Her Piko Piko Hammer she will have to Jump, whirl, and dash through South Island to put a Stop to Robotnik's plans.