Well, it's not exactly free, but it is a great low-cost experience. And most importantly it enables you to get your bearings in the city. For the price of a public transport ticket, you can make your own tram tour of Milan.
If you get on the n°14 tram, which connects Lorenteggio to the Monumental Cemetery area, crossing some famous districts - Chinatown, Brera, the city centre with the Duomo and Galleria, and the Navigli - you will stop close to a lot of the city’s major tourist attractions.
Or, for a tram trip with a retro touch, you can catch the historic n°1 tram which crosses via Montenapoleone, then passes in front of the Teatro alla Scala and the Arco della Pace.