Creatine monohydrate is the most science-backed sports supplement in history and its accolades make it one of the best bangs for your hard-earned buck.
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Past research has shown that long-term use of creatine is safe and increases strength and muscularity by providing energy substrates and by activating a number of anabolic pathways in skeletal muscle. A very current study, published in the Journal of Strength and Conditioning Research, has shown that taking 20 grams of creatine per day for as little as a week can significantly augment anaerobic power.  The scientists were interested in elucidating the ergogenic benefits of short-term (7-days) creatine ingestion (20 grams per day) on mean and peak power (using a Wingate Anaerobic test), body weight, and 1-RM for leg extension and bench press exercises. Upon completion of this double-blinded, placebo-controlled study, the researchers noted a 5.4% increase in mean power with no changes in 1-RM strength or body weight. These data illustrate that creatine's ergogenic effects start showing up very soon after you start using it, further substantiating it as one of the most effective supplements on the market.

Zuniga JM, Housh TJ, Camic CL, Hendrix CR, Mielke M, Johnson GO, Housh DJ, Schmidt RJ. THE EFFECTS OF CREATINE MONOHYDRATE LOADING ON ANAEROBIC PERFORMANCE AND 1-RM STRENGTH. J Strength Cond Res. 2011 Sep 14. [Epub ahead of print]