I give mine Glucosamine. It's liquid form, not sure how it tastes, but I add it in to their food and they gobble it right up with everything else.

It works wonders for their coat growth too!
I actually need to find another supplement for Trixie. She is allergic to shellfish--we found out the hard way a while back when she snapped up a piece of dropped shrimp--and I have not been able to find a supplement that does not contain it that I am happy with.
I am very interested in any suggestions as well.