Ring
From Sonic Retro
The golden rings featured in all Sonic games act foremost as a sort of shield, and also a necessary element for Sonic to become Super Sonic and maintain the state. In later games they feature as a form of currency, as well as being integral to several of Sonic's upgraded moves, like the Light Speed Dash.
In the first case, the rings the character holds protect him from dying: if a character is hit by an enemy and is holding rings, he drops the rings (unless he is protected by a shield, in which case the shield disappears instead of the rings being dropped); however, being hit while not carrying any rings will cause the character's death. Originally, Sonic also received an extra life for getting 100 rings, and another extra life for 200 rings. In later games, extra lives are also awarded at 300, 400 and further multiples. However, in Sonic Unleashed, only the first 100 rings provide an extra life.
In the second case, one of the conditions to attaining Super Sonic form in the older games is to collect 50 rings. Once in that form, Sonic's ring count falls steadily, and he reverts to his normal form once it reaches zero.
Rings seem to have a higher purpose in the Archie comics. In one issue Sonic is able to use a ring to teleport himself, in another he uses one as a sort of fortune teller.