Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
Firstly I assume you mean 'advertisement supported' rather than 'free' (very little is truly free). It entirely depends on your location. In the UK some of the main services I can get are ITV Hub, All 4, Demand 5 and UKTV Play ad supported. There are many others, but these are the big stations. I can also get BBC iPlayer at no extra charge, as I have to pay the licence fee anyway to have a TV and get the OTA BBC stations.
As Andy mentioned, the majority of the channels can be downloaded and installed for free, but you may need to subscribe to a service to actually view anything. And the country you are located in also determines what might be available. If you are in the United States, the "Roku Channel" (yes, that's its name) offers a lot of content for free, with limited advertising interspersed in the video. But it's only available in the US. There's also Crackle, and of course YouTube. Others like Netflix and Amazon Prime are available in other countries, but not everywhere. And even if they are available in your area, the content might not be the same as what's available in other areas. Using something to fool the system to think it's located in another country is illegal and against Roku policies.
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.