I started growing the root beer plant (Hoja Santa) in Houston for shading other plants. It grows profusely (spreads via roots like a vine) and is good for many things. The Mexicans use the leaves for cooking fish and tamales. You can also use the leaves, which get very large, for making concrete leaf molded garden accents.
http://www.bing.com/images/search?q=root+beer+plant&qpvt=root+beer+plant&FORM=IGRE
Lemon balm, citronella and various mints are other useful herbs. Being a chilehead, I always grow as many different varieties of hot peppers that I can. Once found a huge chile tepin plant growing outside my fence in Houston (the birds had drop planted it). You can find these pepper plants growing wild if you know where to look. The peppers make great hot sauce.
I stated growing wasabi last year. They are looking real good for a harvest this year. I have been harvesting the leaves all along and found they don't develop the flavor till the second year.