Icbit.se - you can buy a bitcoin now ($640), and sell it for 20% more in December ($769). For the past month or so their October contract has been trading for 8-10% above market value, and the December one for 15-20%.
This seems great - it's a guaranteed return of 40% APR (minus any fees). However I'm not sure I trust icbit.se. I've got no experience dealing with them. They have been around for a year - so they might be worthy of trust. Does anyone know?
I've been sticking with Bitfinex as they offer a roughly similar service and rate of return.
If those are contracts, then there is the chance the when you sell in December (for what seems like a profitable trade now), the price is higher than the contract price and you are selling way under spot then. Something to consider.