Well, I could suggest Echinacea, but you may as well swallow your money.
Seriously though, the best things (with studies to support them no less) are antioxidant Vits C and E, and also chicken soup. Apparently, the chicken soup contains high levels of certain amino acids (cysteine perhaps), which help with a cold. Apparently, taking both of these can reduce the duration of a cold by up to 40%.