L-theanine is my personal favorite. Helps you focus and reduces anxiety without turning you into a retarded zombie like RX anti-anxiety meds do. Also makes for a bitchin' confidence increasing combo with alcohol...
*edit* Also, as far as I know, amino acids (which L-theanine is) carry no tolerance potential.
*edit* Also, as far as I know, amino acids (which L-theanine is) carry no tolerance potential.