'Sonic 1: South Island Expedition' (2024) by RadiantNexus Team (Hack)
seeks to reimagine the entire game for original hardware whilst maintaining the original core design and introducing new and old concepts.
https://shc.zone/entries/contest2024/1142

'Sonic Ultra 2' (1999) by Gregorsoft
https://archive.org/details/SonicUltra2

'Sonic: Powered Up' (2020) by Flash_G (Demo)
https://www.sonicfangameshq.com/forums/showcase/sonic-powered-up.754/

'Tailwinds' (2024) by metalharbor (SRB2 Mod)
A Pilotwings-inspired mod!
https://www.sonicfangameshq.com/forums/showcase/tailwinds.2180/

'Sonic Conquest' (2024) by Team Hotaru (S2RSDK Mod)
A brand new Zone, and new bosses!
https://shc.zone/entries/contest2024/1098

'Sonic Megamix' (2008) by Team Megamix (Hack)
An extensive reworking of the original game, featuring new characters, art, music, abilities, layouts and more.
https://archive.org/details/sonic-megamix

'Sonic Totem' (2021) by @SonicTotemGame (Demo)
Three whole acts are available in this demo, check it out!
https://www.sonicfangameshq.com/forums/showcase/sonic-totem-sage-21-demo.957/

'Sonic 1 SMS Remake' (2019) by Creative Araya
A widescreen recreation with new stages, unlockable characters, and can be played on mobile!
https://sonic-sms-remake.blogspot.com/p/one.html

'Silver Sonic The Crystal Islands Refreshed!' (2024) by Roebloz (Hack)
An expanded and cleaned up version of this Master System hack featuring Silver Sonic.
https://shc.zone/entries/contest2024/1060

'Sonic 2: Rise of Mephiles' (2021) by Full Sawfish (Demo)
https://sonicfangameshq.com/forums/showcase/sonic-the-hedgehog-2-rise-of-mephiles.938/