eToro let’s you short BTC, Dash, Ethereum, LTC and XRP, though no leverage is available for those, and the spread can be a bit daunting.
Disclaimer: I don’t usually trade crypto there, this is not investment advice.
EDIT: apparently it’s not available in the US.
If you sign up with my referral link we both get $20: http://etoro.tw/2BmqxQb