Watching TV in your media center can't even be called state of the art anymore, it's a must have. For watching Free TV, it's not even necessary to buy any DVB sticks or place your media center close to your SAT wall outlet. The only thing necessary is a stable and decent Internet connection.
First of all, I'd like to mention that this post is mainly focusing on TV stations in Germany, so I can't speak for any other regions of the world. I will strictly speak about what worked for me. Secondly, I know that there are many tutorials out there. Any search engine will give you a multitude of (video) tutorials on how to enable IPTV in kodi/XBMC/OSMC. I will focus exclusively on OSMC, but I assume for the other derivates it's a similar process.
My problem when following these tutorials was that none of them worked out of the box for me, although in retrosepct I understand that there was always only a small portion of what tripped me up. For instance, I never found the enable option for PVR / Live TV in my version of OSMC, leading me to think I'm doing domething wrong. Turns out, skipping that particular step was the only thing necessary.
Okay, let's get some (IP)TV channels on OSMC. In OSMC, this is done via the PVR (Personal video recorder) functionality (yes, I know, it's the kodi wiki, but the description is still good). To use this functionality, there are two steps to be performed: enable a PVR addon and configuring it correctly.
To enjoy IPTV, an addon called IPTV Simple Client will do the trick. It has to be enabled in Settings > Addons > My Addons > PVR Addons by selecting it and choosing Enable. This should automatically bring two new menu points to the home screen: PVR & Live TV as well as Radio. There was no need for me to enable Live TV in the settings, this was done autmatically.
At the same settings page of the IPTV Simple Client, the addon can be configured. You need to provide a playlist in m3u format containing the URL and additional information of the IPTV streams. Unfortunately, most of the lists out there I've tried didn't work for me as they seem to be not very well maintained and URLs are changing.
Kodinerds has a thread providing a list of IPTV channels, explaining in great detail which list provides which channels. The whole list is maintained on github and provides approximately 230 TV channels and 190 radio stations. As a bonus, the lists are accessible via shortened links (which is a big plus when "typing" via remote) and subsets (e.g. only local TV channels) are available.
These URLs worked out of the box for me. OSMC needs some time to load the channels, but the streams work auite well (they are, in fact, the same you watch online). Enjoy! In the near future, I will explore the combination with EPG for added convenience.