Hamburg's Reeperbahn rivals Amsterdam's Red Light District for its magnitude. Located in St. Pauli, the Reeperbahn is a corridor of sex shops, strip shows and brothels. And although the street is free to tour, any of its shows and "services" are very expensive. Some TripAdvisor users specifically note the Reeperbahn's seediness, drunks and even street fights.
But the area in and around the Reeperbahn is also the site of much of Hamburg's nightlife, from pubs to clubs and theaters too. You can tour this area by hopping off the St. Pauli U-bahn or the Reeperbahn S-bahn stations.