In my opinion there is no "not backed by anything". The current 'free-floating' cryptos are mostly backed by trust in the crypto-space in general, which is wildly volatile, as partially reflected by the market value of BTC (arguably). A crypto backed by a commodity like gold would provide an alternative to a "stable value" crypto that tether wanted to be.
RE: Some thoughts about Tether and probably a safer alternative - Bitshares' bitUSD