I went on a Piracetam (Nootropil) phase during high school for about 3 months. I would not qualify the effects as subtle. They were quite dramatic in my case. Razor-like focus and amazing retention of study material. No side-effects that I can remember.

It's been many years, and I would like to self-experiment again. Where can a U.S. citizen get Piracetam from these days?