There are a lot of these "blank" character in Sonic 1 hacks out there. Some are better than other, while some are just downright weird. I've never seen them all in one place though. Time to do something about that.
Knuckles the Echidna in Sonic 1
The one that started it all... and Sega said they couldn't do it.
Tails in Sonic 1
The next logical choice.
Shadow the Hedgehog in Sonic 1
Emo Sonic FTW!!!
Amy Rose in Sonic 1
The gameplay is totally wild! She just smashes shit with a hammer!!!
Sally Acorn in Sonic 1
Remember her from the Saturday morning cartoon? How many kids did she turn into furries...
Ray the Flying Squirrel in Sonic 1
Remember him? No? He's like a wimpier Knuckles.
Mighty the Armadillo in Sonic 1
Sonic's most underappreciated friend. Why does Sega hate him so...
Charmy Bee in Sonic 1
HE CAN FLY!!!
Vector the Crocodile in Sonic 1
Metal Sonic in Sonic 1
Hey wait a minute that's not Sonic the Hedgehog...
Somari the Adventurer in Sonic 1
The tile screen's a lie, the hack is fun.
Eggman the Dictator in Sonic 1
Morbid obesity won't hold him back. Loop the loop you rotund bastard! Loop the loop!!!
Motobug the Badnik in Sonic 1
IDK, it's not really completable, but someone made it...
Stella the Cat in Sonic 1
Fanmade cat character.
Did I miss any? Also, it would be cool to know how many of these work on real hardware. I imagine most of them do, though I heard the Vector one doesn't. If someone with an everdrive wants to test them that would rule.
... someone should make a Sonic 1 X characters hack that has all these guys in it, or at least the decently playable ones (Sorry Motobug).
EDIT: Updated the link to Stella the Cat since the original link is dead.