Formed from a hardfor of the Uniswap protocol, Sushi Swap (SUSHI) is a freely-tradable token with the majority of its volume on major exchanges centralised exchanges. You can contribute to Sushi by adding to the liquidity pool.
When you exchange SUSHI for ETH, a portion of it goes to active liquidity providers, the other portion is converted back to SUSHI and rewarded to holders. Pairs against other cryptos and stable coins are active as well as with fiat, included on Bankman Fred's FTX exchange.