Solo Traveler World occasionally re-runs this informative post describing no- or low-cost ways to get a tour of a city when you travel.
One of the sources listed is Global Greeters. This is a network of individuals in 25+ locations around the world who will show you their city free of charge.
Another idea is the “free walking tours” that you can find online. Note, however, that the guides on these tours are charged (by the website owner) for every person who shows up for a tour…therefore, you should tip your guide generously so they don’t actually lose money.
While the blog post lists 7 sources of free guided tours, one of the commenters adds an eighth source…couchsurfing.