(This post is for historical information only. Download ROMs etc at your own legal risk as the legality is grey area.)
If you are just getting into retro gaming, an ambitious game that came out long ago was is Sonic the Hedgehog Megamix which looks like it hasn't been updated in 12 years but the Megadrive/Genesis version is stable. I'm assuming the Sega CD version they were working on is unstable and likely an abandoned project. The legalities surrounding this game and the ROM are definitely in the grey area.
Original music and 5 playable characters with new layouts make it one of the most ambitious homebrew projects that ever came out for the Genesis platform.
At one point a physical release by game pirates in Russia occurred and was widely distributed mostly on eBay.
*I cannot encourage you to download ROMs as they are grey area legally. This post is for informational and archival purposes.