Rain cape. Almost all the capes out there are similarly cut, whether it's Carradice, Brooks Oxford, etc. I had one from MEC called the Cruise-Aider cycling cape. Definitely prone to wind and billowing, despite its waist strap.
But I discovered the Impac Cycling Cape, which is cut high on the sides. The front is longer than the Cruise-Aider and has a clear "window" to allow for handlebar lights. My experience so far is no billowing whatsoever. I still have all the benefits touted for rain capes: decent rain coverage, excellent ventilation. Below the knee does get wet, but booties will take care of that.
http://www.impaccycle.com/html/0901_cycling_cape.html