The Internet Movie Database: probably the best, and certainly the most comprehensive, site for film info, stills, reviews, etc.
British Board of Film Classification: lots of useful info, including search facility for certificates, running times, etc.
Empire magazine: good reviews and info
Screen International: weekly trade paper, with reviews and film news; some sections need a password
Film Distributors Association: useful for info about future releases and film distribution in general, including links to distributors' websites
Pearl & Dean: lots of useful info about films and future titles