The only supplement I take regularly is creatine. Creatine works by two methods. The first was already mentioned and that is fluid accumulation in muscle which makes them appear larger. This is a cosmetic effect. Creatine also increases muscle performance which is a measurable physiologic effect. Here is a quote from the US National Library of Medicine and the National Institute of Health:Several high-quality studies have shown an increase in muscle mass with the use of creatine. However, some weaker studies have reported mixed results. Overall, the available evidence suggests that creatine does increase lean body mass, strength, and total work. Future studies should take into account the effect of different individual fitness levels of study subjects.http://www.nlm.nih.gov/medlineplus/druginfo/natural/patient-creatine.html
Creatine increases the levels of ATP (energy) for muscle use. It works well for anaerobic muscle function (short bursts of activity like weight lifting, sprinting). It has not been shown to significantly increase muscle endurance so there is no performance enhancement with aerobic activities like jogging or long bike rides.
I have my own anecdotal story about creatine. About three years ago was the first time I used it. I was hoping it would increase my strength at the gym. I also ride a mountain bike. Suddenly I was able to go climb hills with my bike that I haven't been able to do since my younger days. I have no way to explain this sudden increase in performance other than for creatine. Short hill climbing would be an anaerobic activity.
Creatine is more effective the older you are. I'm going off topic now. This is only for the guys interested in the science of aging. One of the theories for aging is the decline of ATP levels as one ages. Creatine would help reverse this effect. There is active research going on with creatine as an aid in preventing sarcopenia( the loss of muscle mass that occurs in all elderly individuals), Alzheimer's disease, and congestive heart failure