When Usher released the video My Boo in 2004 rumors were going around that he and Alicia Keys were dating, while they both denied it, people noticed how much chemistry they had, however many people still assume they dated.
Zooey Deschanel and Joseph Gordon-Levitt were in two films together Manic and 500 days of Summer, there chemistry was too much that many thought that they would be a great couple in real life, making up for Tom's sad ending, however JGL said they were both friends.
Even though in my crazy delusional mind I think he's being friend-zoned.
Leonardo Dicaprio and Kate Winslet broke millions of hearts with Titanic and again with Revolutionary Road, they seem like great friends but there chemistry is amazing.