Arugula - [Eruca sativa (garden), Eruca vesicaria (wild)]
Arugula is the North American name (possibly derived from an Italian dialect) for garden variety Rocket, and no yuppie would consider a salad complete without it.

Chae
This herb is sold as "Chae" in a local market serving mixed ethnic communities (Near East, Middle East, the "Stans", Russia, Persia, India, Mexico). By taste and appearance it's a varity of Rocket, but the market clearly diferentiates it from Arugula (which they rarely have). I've not been able to identify it to genera or species, but it's a perfectly good (and lower priced) substitute for Arugula.