This link leads to the downloads page on the official Team Megamix site
With humble beginnings in 2004 as a music/palette/layout mod of Sonic 1 by a single person, this project has grown into a massive total-conversion effort by an entire team. After trading talents with the original author for our own personal projects, we soon found ourselves leading the newly-formed "Team Megamix" in producing the most advanced Sonic 1 mod to date. As lead programmer, I brought this title to MegaCD using my original PoC as a template, and with my peers, have completely transformed it with enhancements from other Sonic titles and our own imaginations. With 5 playable characters with unique abilities, a well-varied FM soundtrack built from select classic games for current releases and a brand-new CD audio soundtrack on the way, new levels and bosses, and even more exciting features, we take great pride in this continuing project!
For more information about this game and the team behind it, see the official Team Megamix site.
For a little more background and some technical details, see this blog entry.