The specialty area of United States Revenues may be the most fertile collecting specialty for collectors. Scott lists the general issue revenues which are interesting enough. But it is in the field of the specialty revenues where collectors have had the most fun. There are specialty Revenues for everything from Perfume Tax Stamps to Consular Office Revenues (which paid the tax on things like passports and visas), to even Marijuana Tax Stamps (which paid a tax on the illegal drug). Incidentally, Marijuana Tax Stamps were declared unconstitutional by the the Supreme Court as a form of self incrimination since it was illegal to sell marijuana without buying the stamps and paying the tax. But since it was illegal to sell pot anyway, this was ruled law enforcement double dipping. There are over thirty specialty revenues that the Scott catalog lists, and scores more of Scott unlisted revenue types— most of which are called "Tax Paids" and are excluded from the general catalog on the theory that "Tax Paids" represent receipts for taxes that have been paid and thus are not really stamps in the traditional sense as bearer certificates that indicate prepayment of a service that is due the bearer.
Among the more interesting specialty revenues are Revenue Stamped Paper. The Civil War and late 19th Century was a time quite like our own in terms of national tax policy. The goal was to have very low national tax rates. But the demands of government, even then, required revenue, and this was made up by a large number of specialty taxes and user fees. Before 1913, and the sixteenth amendment to the Constitution, progressive taxation was illegal so the government couldn't impose a fair income tax and had to scamper elsewhere for tax revenue. For a while there was a government tax on checks and bonds, and this was paid for by printing the check or the bond on paper which had already been imprinted with a tax stamp indicating that, by buying that stamped paper, the buyer had already paid the tax. Revenue Stamped paper comes in hundreds of varieties of stamps and can be used on a wide array of documents. Many of these are ornately and beautifully engraved documents that can be framed and look great in a stamp den (I have one in mine).
Collectors further specialize their Revenue Stamped Paper collections by collecting by type of document that the revenue is printed on, sometimes by company or bank, and others collect by theme on the background engraving that many of the checks and bonds have. The most delightful thing about this specialty is how inexpensive it is compared to the rarity of the material. Sure, there are rarities that sell in the thousands of dollars, but most items, though seldom seen, sell for only a few dollars each.