There is probably no harder-working community than the one that worships Sega's mascot. Sonic the Hedgehog is a legend. Fans have made everything: comics, games developed from scratch, a lot of fan-fiction literature, and, of course, hundreds of mods of the classic games in the franchise. Let's look at some of these mods here!
It would be impossible for me to cover every single modification for Sonic 1, Sonic 2, and Sonic 3. There are so many! That's because the fan community, as already said, has always been (and to this day) super active. Year after year, "modding tournaments" are organized, where fans show both their love for the franchise and their talent at opening ROMs and changing things to make a somewhat different game.
There are all kinds of mods: ones that change the game completely and ones that just add a different character to the same old levels, with their own powers and abilities. The image of Sonic with bleeding eyes, for example, belongs to Ordinary Sonic and... ordinary it is not; it's a creepypasta of the finest kind! Want more? Well, here's a bunch that will be almost impossible to digest! If you're interested in playing any of these mods, just search for the ROM on Sonic Retro or play them online from SSega (one of my favorite pages on the whole web and the one I visit most certainly). And don't forget to play Sonic Time Twisted!
One of the most striking mods features Lilac, the main character of the game Freedom Planet, in an excellent mod. Freedom Planet, in case you didn't know, is a beautiful tribute to Sonic, which can be bought on Steam and was developed by people who were working on these mods.