Alex-668179 (8/26/2010)
AFAIK, NULL is the only way to leave a field empty!
I disagree: setting a field to NULL does not 'leave [or make] a field empty', instead it makes (or should make) any field value, and even the existence of such a value, undefined, inaccessible and irrelevant. That is what I meant before when I wrote before that NULL is a state: just stop thinking of NULL as a value!
I actually agree with most of that except for the "any value" part, a field cannot be any value or any logical operation on it would always return TRUE, replace any value with "an unknown value" and your definition is 100% correct.
The use of the term blank or empty field comes from thinking of the stage when the data is inputed, normally somebody will enter the data on a form and any fields that are unknown at the time (inaccessible) or not applicable (irrelevant) will be left blank or empty, when the data makes it to the database one way to express that the field was left blank is to set it to NULL, there may be many cases when normalization or some other method is a better choice, but in many cases using NULL is the best choice.