Since we didn't need many thyme plants, we just bought a single plant at the Farmer's Market. In the fall we cut it back and freeze the stems to use through the winter (when we think about it).
The French tarragon is supposed to be the best. Even though we like tarragon, though, we rarely think of using it. We're trying to increase our use of herbs and spices this year.
We tried fennel for a year or two, but we didn't like it that much. (It is another plant, though, that black swallowtail butterflies can use as a host plant.)
Our celery experiment
We decided to experiment with regrowing celery. I expect it won't develop big stalks, but even if we get some thin stalks, it's easier than growing it outside. Still in the early stages of the first experiment … (see sidebar for more info)
UPDATE: We didn't find this to be useful. And where would the nutrients come from to keep producing the plant? It was interesting to try, but didn't work out for us.