Tour guiding
Officially, tour guides in Rome need to have a license. The police in Rome are apparently trained to identify tour guides and do occasionally pull people over. Not having a license means an instant fine. The license itself is difficult to get hold of, requiring you to sit exams which are held very rarely, and it Italian.
This was highlighted in 2004 by the case of Giovanna Pizzorno, who was fined €172 for apparently 'pointing out the historic and artistic beauty of the Trevi Fountain without authorization.' You can read an article about this here. In 2005 The New York Times carried a story about the crackdown on illegal guides, which you can read here.
That said, there are always companies willing to employ unlicensed guides, especially in the busy season. Whether you want to risk getting into trouble is your decision, but you'll find vacancies listed - often thinly disguised - on sites such as Wanted in Rome.
