Open box means that someone bought it, opened it, probably used it, and then returned it because they didn't like it (remember, satisfaction guaranteed!) or maybe it had some problems.
I have a phobia of buying used parts online unless the deal is insainly good and the part has a return policy that if it doesnt work I can return it (open parts have this type of warrenty ?) I was looking at a X1800 for 156$ open box.... I didnt buy it but maybe I should have I dont know ! *shrugs*
Edit:256MB's if that makes any difference on weather or not it was a deal.