One time a friend and I were trying to remember something; I got it two days later and called. All I said was the word, he said, "That's it!" Then we both hung up.
One summer, I was working and rooming with a girl that I'd completely hit it off with. She and I could somehow have entire conversations in our heads because our thought processes worked the same way. Some one else would say something that lead us to thinking and then one of us would say something that the other person total understood but was a complete non sequiter to everyone else.
Everyone else would look at us and then usually, the person who didn't speak would explain the thought process going on. And everyone else would just shake their heads. It was especially bad when our thought processes lead us to something funny and the two of us would just break out laughing like loons when no one else was.
Our lives have completely diverged; we don't even send Christmas cards anymore but for that summer, it was amazing.