I've been hitting the gym pretty hard lately. One thing I've been using is NOXplode, a pre-workout supplement that's helping me get some results. Its primarily nitric oxide, and what it does is dilate your blood vessels, allowing more oxygen to flow to the muscles during exercise, and is especially noticeable when weight lifting.
If you try it, you'll definitely notice it gives you a good boost while working out. Although if you do try it, I STRONGLY ADVISE AGAINST taking it with a full stomach. You'll feel terrible.