There may be different option to get free museum passes I would like to share the information which I came to know recently through a friend of mine..you all can also share if you know something different.
1) Either by their free day or company free entry…
2) I guess everyone must be knowing about museums through bank of america where first weekend of the month entry is free if you are member of Bank of America.
And its at many Places in USA…more details on
http://museums.bankofamerica.com/
Recently I came to know that we can get total 2 passes free per calendar year for 2 different venues per card holder 18 and up. ( per card 1 adult and 1 child) through our local library ( not all city library might have but milpitas library has) you just go to your local library website search for museum passes put your library card and pw and then you can view the venues available for musuems , dates and can reserve it.
California Academy of Science museum has some days free but thats only for the near by residents or according to their Zipcode.
Thanks Poonam for reminding about library passes, remember seeing it or hearing about it but have never used it.. .
Exploratorium has few free days in a year for everyone..
California Academy of Science has few free days for SF resident. But they also have free days for everyone