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.

Sonic R
First appearing in Sonic R, during Grand Prix each of the courses have 5 Sonic tokens. Collecting all of these tokens while comming into first place would allow you to race an unlockable character. Once you beat the new character unclocked.

Sonic Adventure
Reappearing in Sonic Adventure, this time with the Sonic Team logo on them. 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 and pile up in the vault.

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

SegaWorld

 * Wikipedia article: SegaWorld

SegaWorld at Sydney also used Sonic tokens, these were used to play the prize/ticket games in Segaworld. 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.