monthly museum passes

For what and where and when? As a for instance, you can join "Friends of the Louvre" for 70 euro, or 110 euro for a couple. Its good for a whole year. http://www.amisdulouvre.fr/participer/adherer.htm Use google translate to read all the benefits.
I'm sure most individual museums have similar programs.

Which city or country are you considering? For the Netherlands, there's a Museumkaart which is only about €55 for the whole year, good for many (but of course not all) museums in the entire country. A fantastic value - if you're going to be in the Netherlands for more than a few days.