Their seem to be several "official" websites to many of Florence's museums and churches. Sometimes it is just overlapping bureaucracy (the Commune may have a site, the state may have a site, the museum itself may have a site), and sometimes it is people pretending to be an official site, either to try to get you to click on an ad or book a hotel or something else that either makes them money or brings their website traffic. Then there are the hundreds (if not thousands) of small sites who have their own listings and descriptions of some or all of the museums in Florence (like this one!). Below are links and brief descriptions to the official sites of the major museums of Florence:
Firenze Musei is a network of thirteen state museums in Florence (Accademia Gallery, Uffizi Gallery, Palatine Gallery, Silver Museum, Gallery of Modern Art and Costume Gallery, Medici Chapels, San Marco Museum, Bargello Museum, Cenacolo di Andrea del Sarto, Davanzati Museum, Boboli Gardens, Archaeological Museum, Museo dell'Opificio delle Pietre Dure) housing works of art from most of the collections of the Signori who ruled Florence (the Medici and the Lorraine families) as well as some works originally belonging to ecclesiastical bodies that fell into the hands of the State as a result of the 19th-century suppression of religious bodies. The site has location information, opening times, ticket prices, etc. Unfortunately the events page is weak and not maintained (the last thing listed was a special exhibit that closed in January of 2003!) but the phone numbers should be valid. If you are planning to visit any of these museums on a tight schedule you really should confirm they will be open etc. with a phone call ahead of time.
This site has now been merged/morphed into the one mentioned above.
The Commune of Florence also maintains a home page for their museums (Italian only), the Musei Fiorentini. Some of the museums/churches on this site are the Cappella Brancacci, (Brancacci Chapel), the Museo Stibbert, the Museo Bardini (which is currently closed for renovation) and the Palazzo Vecchio. Opening hours, phone numbers, ticket prices etc. are all available on the site.
The third major website is the Polo Museale Fiorentino, a repetition of the first one (the state museums), this one again run by the local government of Florence it seems. They seem a little bit better about updating the news and current events schedules on this site, but sometimes most of the details are in Italian only. This site covers almost all of the same museums as Firenze Musei does, with the same kind of information - phone numbers, opening times, locations, etc.