OKCoin.com is the most interesting right now.  They are just getting it off the ground, so its slow, but they are working on getting their API up and running, so MarginBot will be supporting it soon.  At this point, their traffic is a bit too low to be useful, but I think this will likely change, especially if BFX doesn't quickly realize how badly FRR is hurting their platform, and people start moving their margin investments to greener pastures.