The Frankfurt Card in Frankfurt is a decent deal. For one person for one day it is 9.90 € and offers transportation all day including travel to and from the airport and 50% off entry to 28 museums. Transport itself would cost 8.85 € for one person. A 2 day card costs 14.50 €. A group card for one day is 20 € (transport, including airport for a group of 5 would be 15.80 € for just 1 day) or for 2 days, is 29.50 €. These are decent savings.
If you are already here or want to visit a lot of museums in one or 2 days, then the museum card is a good bargain too. Ticket for one person is 18 € and is valid for 2 consecutive days in 34 museums. Family tickets are 28 € and good for 2 adults & 2 kids under 18. Again, this is valid for 2 days. Considering that museums like the Städel cost 14 € p.p. those tickets pay for themselves even if you visit just 2 museums.
http://www.museumsufer-frankfurt.de/portal/en/News/Start/0/0/0/0/2360.aspx
Do remember, that in Germany, most museums are closed on Mondays, so it is always good to check if the museums you want to visit will be open during the time period you are in the city. Also, lots of museums close down for up to 1-2 years for extensive renovations. We have 2 closed right now, the Judengasse Museum and the Money Museum and a 3rd one, the Jewish Musuem getting ready to close in July.