Tarragon
A fresh herb with narrow leaves and a distinct anise note, often used by the bunch in Georgian cooking.
Fresh tarragon turns up only occasionally in larger supermarkets; farmers' markets, Turkish or Eastern European grocers and garden centres selling it as a potted plant are more reliable sources. Dried tarragon is sold everywhere but only works where the herb plays a minor part.
Substitute:
Where tarragon goes in fresh and in quantity, there is no good substitute, since dried tarragon tastes flat and hay-like. Chervil or fennel fronds come closest to the anise note but are milder and less pungent.