Where to advertise a free service to filmmakers

Where are you located, and is the service location-dependent? Most places in the US that have large filmmaking communities have specific local resources that disseminate information of the sort you want to convey. Most places with a film community of any size will have a local government film office. Sometimes this is organized at the state level, or in major cities it's a city government thing. Googling "[Place] Film Office" will help you find these resources. There are also trade publications that advertise film services, though this is more likely to be in the form of paid advertising or buying a listing. Mandy.com is a thing. In my opinion it used to be a lot more worthwhile a few years ago, but you can definitely throw up a listing and see if you get any hits. There are a few other sites akin to Mandy (Stage 32? Staff Me Up?), but nothing that's universally used throughout the film community. Meetup might be worth looking at, more as a networking tool than as a place to spam info about your service. But some groups are totally into having service providers come in and do presentations. There are also really granular sites, google groups, listservs, etc. catering to specific niches within the industry. I'm a coordinator and use various production services/coordinator specific websites. There are also sites for location managers, camera people, grips/electrics, editors, art directors, actors, screenwriters, et al. I would suggest getting a very specific idea of who is going to use the service you're offering and find out where those people go on the internet. If you're a dialect coach, there is no real point in adding a listing to a site for grips. The various entertainment industry unions might also be helpful to you. People love useful free services, and it's easy enough for the union to throw up a link to your website on their "resources" page. This is another area where you'll want to get a very specific idea of who your audience is, and do some research to figure out what union or guild represents them. Note: I'm really not sure what you mean by "regarding the content of films, rather than the filmmaking process". It would be a lot more helpful if you could describe what the actual service you want to provide is. Do you mean like script coverage and notes? B roll? Are you trying to connect directors with writers and actors? Do you do research? Most people on the creative side of filmmaking are not really looking for a service to provide content, whether it's free or not. The content comes from inside their brains. If you happen to be located in Los Angeles, you basically can't spit without spattering some kind of meetup group, networking project, listing service, sourcebook, etc. relevant to the entertainment industry. I've actually seen vinyl signs draped over the sides of buildings offering them up as potential filming locations. There are flyers for actors' workshops on every street corner.

Where are you located, and is the service location-dependent? I'm in the UK, and the service can be carried out electronically. No physical presence required.

http://mandy.com/ is hugely popular, both in the US and UK.

There are some subreddits dedicated to filmmaking: http://www.reddit.com/r/filmmaking http://www.reddit.com/r/filmmakers