Steem Dollars, or Steem Based Dollars, are stablecoins released by the Steem platform and pegged to the USD. Steem Dollars (SBD) are one of the three currencies that help power the Steem social media platform. SBD holders are entitled to receive one dollars worth of STEEM when it is redeemed.

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.

The Maker token (MKR) is the MakerDAO native token. It acts as a utility token, governance token and a recapitalization tool for the MakerDAO platform, which manages the DAI stablecoin. MKR is used as part of the stabilizing mechanism that is in place to combat the price volatility of the DAI stablecoin.
DAI is pegged to the US dollar and has collateral in place to ensure stability. The MKR token is used is to pay for the "collateralized debt positions" (CDPs) that have been used to generate DAI. After this MKR is used, it is removed from the supply. MKR holders also contribute to the governance of the MakerDAO and vote on crucial issues related to the direction of the DAI stablecoin.