Panax Ginseng is the main one. Others that are beneficial for stimulating free testosterone are Chrysin (a number of sources including the passion flower and pelargonium), Nettle root, Muira Puama (from the stems and roots of the Ptychopetalum olacoides plant), Yohimbe, Saw Palmetto berries, Gingko Biloba, Tongkat Ali (Eurycoma longifolia), wild Mexican yams.
Check out this site for interesting stuff on testosterone/libido enhancers for aging men (I know none of you young bucks need it - remember it for your dads, older brothers and yourself after 40, heh). http://www.lef.org/magazine/mag2000/jan00-cover2.html
On the other hand, soy contains phytoestrogens and so lowers testosterone...(I once had a teacher who said the similarity in shape of the soy bean to testicles meant eating soy could shrink testes to their size - anyone want to test that theory?).