Sonic Token

Sonic Tokens are gold coins with Sonic's face on them. They appeared in a few games and used to unlock features or proceed in games, such as Metal Sonic, Tails Doll, Metal Knuckles, and Eggrobo. There were also actual Sonic tokens used in SegaWorld amusement parks and arcades, before those places closed down.

Sonic R
First appearing in Sonic R, during Grand Prix each of the courses have 5 Sonic tokens. Collecting all of these tokens while coming into first place would allow the player to race an unlockable character (sans Robotnik, who can be simply unlocked by claiming first place in all five tracks). Once the new character is beaten in this race, the character will be unlocked.

Collect 5 Sonic Tokens for these characters:


 * Resort Island - Metal Sonic
 * Radical City - Tails Doll
 * Regal Ruin - EggRobo
 * Reactive Factory - Metal Knuckles

Sonic Adventure
Reappearing in Sonic Adventure, this time with the Sonic Team symbol on them (which pretty much resembles Classic Sonic's face anyway). They are used in Sonic's story as Sonic needs to collect 400 rings in order to finish Casinopolis. When Sonic makes the deposit (by a switch, mechanical pincers will grab and violently shake the rings from him) his collected rings will be converted into tokens (1 ring = 1 token) and pile up in the vault.

Knuckles can see Sonic's tokens on his stage of Casinopolis as well.

SegaWorld

 * Wikipedia article: SegaWorld

SegaWorld also used actual Sonic tokens, these were used to play the prize/ticket games in SegaWorld. London Sonic tokens had one side had Sonic's classic pose while the other side had the Sega logo.

The Sonic tokens in Sydney were more traditional to the games as one side had Sonic's head while the other had SegaWorld Sydney title on it,

Various SegaWorld's also went by Sonic Tokens to play the games.