Have a look at the website www.seefilmfirst.com - you can register for free and receive email alerts for film previews near you. SeeFilmFirst is a ticketing system, used by the major film companies and the media to supply tickets for previews. For some films they have 1,000s of tickets to offer members, for others they have allocations after offering to a third party promoter so it can be potluck how many tickets are available for each film. All tickets are offered on a first come, first served basis.
If you live in London, you can find free films on the LondonisFree website (click below). You can also find free theatre, concerts, etc. on this site.