Donna M. Raditic, DVM, CVA, DACVN recently discussed the use of supplements and therapeutic diets in the management of pets with osteoarthritis (view it On Demand - running time 46 mins).
She also provided us with a very helpful resource that lists several commercially available nutritional supplements. This table provides a comparison of each product's ingredients (some of which were discussed in her presentation) as well as Company information (including whether the product has been tested by the National Animal Supplement Council - NASC).