Ostashkov's Regional Museum is located in what was once the Trinity Cathedral, which was built between 1685 and 1697 in the baroque style.  For such a small city, the museum is rather good with lots of information on the city and its citizens, including mock-ups of a traditional Russian izba (wooden house), a merchant's house and a blacksmiths.

The bell tower is also part of the museum and you can go to the top to get great views of the city as well as look at the small exhibition here of bells and bell towers in the district.  In addition to this main building there is also an exhibition hall on 19 Ulitsa Volodarskogo which holds various temporary exhibitions.