Revenue stamps are all sorts of stamps issued by governments or government-related organisations to pay taxes or services other than mail or telegraph services.
Canada (1897): Electric light inspection stamp
Canada (1897): Gas inspection stamp
USA (1898): half cent revenue stamp
USA (1930): 2 cents revenue stamp
Great Britain (1882): £5 telegraph stamp
British passport stamp 1910-36
Italy/Libya (1930): visa Tripoli paid using 5 lire revenue stamp
Swiss visa issued 1931 paid using 5 CHF revenue stamp