St. john's wort
"I have found this herb to be an invaluable part of self-maintenance.  Though I do not always take it, months will go by when I do not feel the need, I have relied on it for over a decade.  I come from a family with a history of depression and am one of the few that has never needed an anti-depressant drug.
One word of caution:  once a bottle of st. john's wort is open, I find that it loses potency with in a month or so.  I'd stay away from large bottles, be careful how it's stored and there are other herbs that compliment it nicely: DMAE (not to be confused with the hormone DHEA!), valerian and such.
Good luck!"