Straight caffiene would be a safe bet. In normal doses it's perfectly safe and has tons of proven ergogenic effects.
Oddly enough, someone died two days ago of caffiene overdose. I'd never heard of it before this guy but it turns out he consumed 15 no-doz tablets at one shot. The normal is 1 every 4 hours
Mike, do you know much about sodium and potassuim phosphates? From what I've read, they seem to work. Do you have any experience/knowledge regarding these supplements?