Several years ago I sat in on a presentation about making synthetic jet fuel from algae. It sounded very promising - apparently the algae would consumed stuff such as sewage waste, and the algae could be easily processed to produce a synthetic oil like substance. It didn't take much area to produce large quantities of the algae (far more space efficient than biofuel from conventional crops such as corn or soybean) although it did take large amounts of fresh water. They were working on finding a suitable type of algae that would allow the use of seawater.