The two I'm familiar with are BitShares and NuBits. BitShares uses contracts-for-difference and price feeds to create stable "BitAssets", like BitUSD, BitCNY, and BitGold. NuBits is a central bank on a blockchain, and it uses its control of the monetary base and interest rates to peg its value to the dollar.

Disclaimer: I own some BitShares. I won't answer more questions here to keep things on topic, so email those to me.