For those of you in the US, every summer Regal cinemas play free movies every Tuesday and Wednesday. All of the movies of G or PG (one of each each week) which makes it something everyone can enjoy. Here the theater is usually not full but there are plenty of moms with kids of all ages including infants (with older siblings). It's completely free, unless of course you hit the concession stand. They are not first run movies of course but most of them are popular, well-known movies like Charlotte's Web.
If you want to see if a theater near you is participating this summer, go to http://www.regmovies.com/freefamilyflicks/ffffresults.jsp , click on REG Free Family Film Festival on the left side of the page then scroll down to your state. You'll get a list of theaters participating in your state as well as what movies their playing and the dates. It seems like most theaters start around June 19 and end around August 20 but it may be different in your area and the movies that are shown do vary somewhat by location.
I am not associated with this chain in anyway. It's just that we really enjoy this program. You can check your local newspapers or your parks association as well because a lot of times there are free outdoor movies during the summer sponsored by different groups.