In Pulp Fiction, there is a briefcase which has contents that are kept very secret. What item is most likely to be in the briefcase?
Roger Avary, the co-writer of Pulp Fiction says:
Originally the briefcase contained diamonds. But that just seemed too boring and predictable. So it was decided that the contents of the briefcase were never to be seen. This way each audience member would fill in the blank with their own ultimate contents. All you were supposed to know was that it was "so beautiful." No prop master can come up with something better than each individual's imagination. At least that was the original idea. Then somebody had the bright idea (which I think was a mistake) of putting an orange lightbulb in there. Suddenly what could have been anything became anything supernatural. Didn't need to push the effect. People would have debated it for years anyway, and it would have been much more subtle. I can't believe I'm actually talking about being subtle.
from Roger Ebert's Questions for the Movie Answer Man
Well, people try and make tenuous Links between Reservoir Dogs and Pulp Fiction.
By that, I mean people try and see if they hold up as sequels, whether continuity-wise they can work. A logical opinion is to say that the diamonds that are stolen in Reservoir Dogs are what are in the brief case in Pulp Fiction.
However, Harvey Keitel and Quentin Tarantino act as two different people in both films. So, this still doesn't hold true. I heard a theory that it was Wallace's soul that was in the case. Evidence for losing his soul is the injury on the back of his neck. However, that seems far fetched.
I've always assumed (despite Avary's explanation) that it was a clever movie reference to the McGuffins in two previous movies.
The most recent being the great Repo Man (Dir Alex Cox, 1984) where a briefcase purported to contain alien remains and which glows mysteriously is a key thread. This, in turn, may have some relationship with the case of radioactive material in the great noir thriller Kiss Me Deadly (Dir Robert Aldrich, 1955).
If you are going to include references to other movies (a known Tarantino trait) those are two awesome movies to reference.
I heard the theory that @JosephLynn mentioned about it being Wallace's soul - and relating it to the injury on the back of his neck. The theory only has slim backing - the glow, the band-aid, the combination of the briefcase (666) and the 'miracle' that happens as Vince and Jules come to take the soul back, perhaps rescuing it from the devil.
Obviously the quote from Avery saying that they didn't have a fixed idea of what it was kind of trumps that.
Until reading the other answers here I always assumes that it contained gold bars because of the orange glow. I know this is a slightly boring conclusion but I don't think that Pulp Fiction is supernatural in any way and the theory about Wallace's soul is just silly.
It had to be something very valuable due to Tim Roth's reaction to it and the colour of the glow makes me think gold rather than diamonds.
It's up to your own imagination
As Quentin Tarantino explains, some things are better left to the imagination than made explicit.
I like the idea that you open up the briefcase in Pulp Fiction, and I don't tell you what's in there. But it's up to you what's in there. And now that's your movie. And you'll make that decision somewhere down the line.
Now, if I tell you at this table what it is, you'll throw that away. And I don't want you to throw that away, that's your movie.