Opposite of brain fart, cerebral blockage, bad idea, and so on.
Fred: "I can't believe this store stops selling beer at 9 pm. What are we going to do?"
You: "Brain wave! The one down on Jeff Davis Hwy sells until 11! Get in the truck and let's roll!"
Fred: "You are a genius!"
Bob: "My car stopped working, how are we going to get to the show?"
You (again!): Brain wave! My neighbor always leaves the keys in his ignition. Let's roll!"
Bob: "You are a genius!"
A brain farth is also some kind of idea.