What a great little museum, I would've enjoyed it even if I'd had to pay! Full of beautiful paintings and little oddities, and the two ladies who run the place are absolutely charming--they don't speak English, I don't speak Italian, they were still wonderful guides because of their warmth and enthusiasm. Truly beautiful paintings and curios throughout this small home, quite a few things we have never seen before.
It can be a little confusing to figure out, we had somebody tell us that the museum is closed with no opening hours. Not so! Go one Bridge up (North) from the bridge to Castello d'Angelo, it's on the city side, on the east side of the Tiber. The Napoleonic museum(also free, surprisingly good!) is on the frontage road, this museum is around the side. There are three doors in the "lobby", take the one in the middle. Doors open promptly at 2:30 PM, tours are around 50 minutes long, toilets are in the middle of the tour if you want them! Very grateful we made the extra effort to figure this one out.