Lemongrass is a common ingredient in good Thai food, but don't try to make do with the dried, packaged version; it's nowhere near as potent as the fresh product. You should be able to find fresh lemongrass at any Asian supermarket. Frozen lemongrass is almost as good, however.
For a uniquely Southeast Asian flavor, cook with galangal (blue ginger). It's related to ginger, but has a piney flavor with overtones of citrus. You can find it in most Asian and Thai food supermarkets, though you may have to make do with frozen galangal rather than fresh.