Washington, DC is one of a handful of cities that requires tour guide licenses. As a guide in DC, I'm required to fill out some forms, pay some fees, and sit down for a written test. Thanks to some recent reforms within the District's Department of Consumer Regulatory Affairs (DCRA), this a relatively painless process. I did it in DC and New York, and am none the worse for wear.
But interestingly enough, this process is under attack. Today, the Institute for Justice, a Libertarian think-tank is suing the city in Federal Court on behalf of two Segs in the City tour guides, alleging that the process in unconstitutional. They even have a snazzy video: