The bright green color of tarkhun is what first prompted us to try this Russian tarragon-flavored soda, but we're glad we did. It has a rather strange aroma of baby powder ("It smells like Infant!") but a pleasant, light citrusy flavor that is very quenching.
There are several brands for sale in the US, but you'll need to find a Russian market first - we've never encountered it in a mainstream supermarket. It's available in 16-ounce bottles, 12-ounce cans, and large 2-liter plastic bottles.