Sonic The Hedgehog: The Lost Worlds is a hack of Sonic the Hedgehog 2 by ExecByte. This hack features all of the zones that were cut from Sonic 2 during development. Some zones have been ported from the Sonic 2 Simon Wai prototype and then finished up, and some have been created based on magazine pictures and information from interviews with people who worked on Sonic 2.
Sonic 3D: No Flickies is a hack of Sonic 3D: Flickies' Island made by Sik. The game remains mostly unchanged; however, the Flicky collection system (part of the core gameplay of the original game) has been removed, allowing the player to complete the levels without needing to collect all the flickies in the level to advance. This makes it much more similar to traditional Sonic games, where the goal is to simply make it to the end of the level without dying.
This hack was later built upon further in the hack, Super Sonic in Sonic 3D, which was also created by Sik.
Sonic 2: Advanced Edit is a Sonic 2 hack created by rika_chou. Sonic 2: Advanced Edit has seen 4 major releases. The 4th release includes the levels from the 3rd release as bonus content.
After putting the finishing touches on his new time machine, the prestigious Dr. Tang steps back to admire his life's work only to see it vanish before his very eyes. Waking up hundreds of years in the past, the doctor's young and careless neighbor soon realizes the shocking mistake he made while playing around in the laboratory. Pursued by hordes of fierce demons, will he survive long enough to retrieve the time machine and return home?
Tails in Sonic 1 is a hack of Sonic the Hedgehog for the Sega Mega Drive/Genesis originally created by drx which featured Tails as a playable character instead of Sonic. Tails can fly, spindash and swim in the same way that he can in Sonic 3.
Robotnik's Revenge is a hack of Sonic the Hedgehog 2 by ColinC10. It is a "boss rush" hack, in which all seventeen bosses from Sonic the Hedgehog and Sonic the Hedgehog 2 are fought in a somewhat arbitrary order over the course of a single level. The game takes place in Death Egg Zone from Sonic 2, and is meant to imply that there is more to the ship than there is in the original game.
Time spent completing the boss run is kept and the total time is given at the end of the course, along with a ranking. In Time Attack mode there are infinite lives and rings that can be found in-between battles. In survival mode, there are only 3 lives and no rings.
This ROM hack adds the Spindash from later Sonic games (and ports) and fix the notorious "Spike Bug", which causes the player to die after falling in a spiked pit.