Tether (USDT) is the largest stablecoin cryptocurrency. It is pegged to the US Dollar. (1 USDT=$1 USD) Each Tether token that is issued is backed by $1 USD that is supposedly held in reserve. Tether has been the subject of some controversy because they have failed to allow a transparent audit of their currency reserves.
Tether facilitate transactions between cryptocurrency exchanges at a rate that is fixed to the United States dollar. This allows traders to take better advantage of arbitrage opportunities. Tether was created by the leaders of Bitfinex, one of the largest crypto exchanges.

Single Collateral DAI is a decentralized stablecoin that has collateral to back its value and an interest rate mechanism to stabilize its price. It is designed to be a digital currency with a stable value, that is pegged to the US dollar. (1 SAI=$1) MakerDAO is the decentralized autonomous organization that manages DAI and works to combat its price volatility. MakerDAO hopes to enable more decentralized applications by providing a stable digital currency whose value will not be susceptible to drastic fluctuations. The Maker platform itself, is powered by the MKR token, which is not a stablecoin, and has a volatile price because of its role on the Maker platform. In November of 2019 MakerDAO launched the Multi-Collateral Dai (MCD), which will eventually replace the existing Single-Collateral Dai, and will enable the company to offer a wider range of DeFi services.

USD Coin is a stablecoin created by Coinbase and Circle. It's backed by USD (1 USDC=$1 USD), and is designed to be a stable digital currency that will fit within the trust and regulatory frameworks of the financial system, while still benefiting from the transparency and advantages of crypto powered networks. USDC is issued using the CENTRE open source framework and membership scheme, and it is available as a framework that multiple companies can leverage. Eventually other financial institutions will be able to become USDC issuers (and issuers of other fiat stablecoins - e.g. EURC, JPYC, GBPC), and customers will be able to tokenize USD and redeem USDC tokens in an open and interoperable ecosystem of financial institutions. A membership governance model for issuers will ensure that they meet technical, operational, regulatory, compliance and audit requirements.