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.

Reserve Rights is an Ethereum-based stable token that is built on the Reserve Protocol. Reserve token will be initially backed by several other collateral tokens that are each tokenized versions of the US Dollar. The project has plans to transition to a collateral model that will not be pegged to the US Dollar.

Dignity is a stablecoin pegged to gold, that was developed by the company Cryptobontix. Each DIG token is pegged to the worth of 0.02445 grams of gold, which was worth exactly $1.00 at the time of the Dignity whitepaper.
Cryptobontix's portfolio of stablecoins are based on a partnership with bullion suppliers. Each individual token acts as a security on a 15-year convertible mortgage. The DIG token is pegged to the value of gold. Every year, 1/15th of the tokens can be redeemed for gold valued at around $1.00.
Cryptobontix does not actually own all of the physical gold. The company plans on spending its profits on acquiring the actual bullion that token holders will have the option of converting.