is it open source? if bitfinex goes down, will tether be able to continue?
Indeed. Birfinex has little to do with Tether other than the love it and plan to use it. If Bitfinex burns up tomorrow, Tether will still be going strong on many other useful places. Tether is build on Omni Protocol. It rocks. You'll see.
What makes tether different from the likes of OKPay?!
Tether uses the bitcoin blockchain.
It is build into Omni (the old Mastercoin platform).