Some of their most famous operettas are H.M.S. Pinafore, The Mikado, and The Pirates of Penzance.
Thanks to a wonderful movie by Mike Leigh called Topsy-turvy, more people may now know about the operettas. It's a good, accurate portrayal of the time and the people.
The word Basingstoke - which is, I think, a city in England - is used in the operetta Ruddigore, as a reminder to a once-mad maid (she had gone insane when her former love became Baronet of Ruddigore, and was doomed to commit a crime a day until he died, but she got better when the Baronet's older brother was found and forced to assume the title) that she should not relapse into madness, and that she should control her behavior.
Those who are interested in the exact phrasing of the scene should take a look at the Gilbert and Sullivan Archive Home Page, which has the full text of all of the operettas, and lots more information about both Gilbert and Sullivan.