Though I doubt "Cinderella Man" will ever be considered a classic. There are lots of "good" movies that don't do well for one reason or another. We're talking about those that have gone on to have a life beyond death.
Citizen Kane is often cited as a movie that bombed critically and financially. As far as I can recall, it did respectable business worldwide and made a profit (albeit not a large one). Plus, it won an Oscar for best screenplay, and was nominated for best actor, art direction, cinematography, director, editing, film score, picture, and sound editing, Hardly the sign of a flop.
The Party was a box office failure upon it's initial release, but is know established as one of Sellers' (and Blake Edwards') definitive films.
Raging Bull was generally regarded as a box office failure.
I don't know if it's considered a classic outside of genre buffs, but Gremlins 2 is regarded by many as being superior to the first film.
I also heard The Producers didn't do well upon it's initial release.
Carpenter has always referred to Assault on Precinct 13 as a box office failure, but is now one of his most highly regarded films (as is Big Trouble in Little China). They're both definitely cult classics at the very least.
On Her Majesty's Secret Service is one of the most highly regarded (and most Fleming-esqse) of the Bond films, and it did relatively poor business when release. It made money, but not anywhere near what the other Bond films did.
Considering that Oz was the most expensive film made up to that time by MGM, they knew it most likely wouldn't return a profit right away anyhow. They knew that they could re-release it in the future.
It's not considered a "classic" by the general public, but it is to me, so I choose "Ed Wood" as my contribution to this thread. I wish it had gotten more love from moviegoers, but I guess it was too weird for that. At least it got Oscars and seems to be more popular today.
One that comes to my mind is "Somewhere in Time", which originally came out in 1980 (I remember going to the sneak preview screening of this film prior to its theatrical release), and it wound up tanking at the box office in its brief run. It wasn't until cable television screenings during the mid-to-late 1980's that it picked up a fan base because of the sweetness of the love story, and now it's a classic fan favorite. It was largely because of this film (and not necessarily because of "Superman") that resulted in Christopher Reeve getting his star on the Hollywood Walk of Fame. And it's even more poignant to watch in the year since his untimely passing.
I'd say most of Carpenter's early films qualify at least as cult classics. He's joked that his movies always bomb at the box office but somehow develop followings after 15 years or so. Assault on Precinct 13, The Fog, The Thing, Big Trouble in Little China, and even They Live and In the Mouth of Madness are more highly regarded now than they were when first released.