So far as I know, caffeine is the only proven performance-enhancing drug that is not sanctioned by WADA.
It is a potent CNS stimulant. However, tolerance builds very, very quickly. This, combined with its social role and free availability, helps us forget how powerful a stimulant it is.
I never take caffeine except for study purposes. But when I do, it boots me directly up the behind. Trust me, when you use it infrequently enough to not build tolerance you can tell that caffeine is more than a little pick-me-up.
It's a genuinely powerful stimulant that is worth saving for when you need a genuinely powerful stimulant.
Like big races, for instance