I only know of lending club, BTCPop, bitbond and BTCjam which no longer takes Americans
Depend on my experiences, too difficult for borrow in BTCPop, bitbond and BTCjam. I recommended to use poloniex for lending bitcoin, the fees or rates is very small, after I lend I use for trading, before we borrow, we should have a reserve fund for the payment terms. I very recommended to borrow poloniex.