I found this article in Cosmetics Business online magazine, which details some of the recent cosmetic ingredients that are claimed to help strengthen hair, accelerate its growth or add the appearance of extra density.

None of these ingredients have been subjected to the same degree of rigourous testing as pharmaceuticals, but if you have a few bucks to spare and you're already using one or more of the proven treatments, then it might be worth looking out for shampoos, conditioners and other hair products containing these additives. The article lists the INCI names of the various substances (INCI names being the things you'll see on ingredients lists) so you can Google to see which brands use which.

Among some of the more interesting additives -

Anageline - Hydrolyzed Lupine Protein

Vital Hair & Scalp Complex - Saccharum officinarum (sugar cane) extract, Citrus medica limonum (lemon) fruit extract, betaine, Pyrus malus (apple) fruit extract, Camellia sinesis leaf extract and hexapeptide-11

AC Volumising Complex - Aqua (&) oryza sativa (rice) amino acids (&) polyperfluoroethoxymethoxy difluorethyl PEG phosphate (&) lactobacillus/phoenix dactylifera fruit ferment extract

Zenigloss - Castor isostearate succinate or Polyquaternium 57.