The most popular supplements (and for good reasons!) are Omega Oils, Probiotics and Glucosamine.
When choosing a good omega oil EPA and DHA concentration as well as taste and form (liquid, softgels) are good places to start.
Probiotics can be a bit tricky as not only the number of live organisms but also the scientific strain - e.g. Lactobacillus Acidophilus - are important. If you need some help just ask Dr. Lifestyle for some personalised advice!
The most exciting development in Glucosamine supplements is that an Italian company has been able to create a vegetarian option that is produced by micro organisms (the only natural source for Glucosamine are shellfish shells)