The most commonly used are: The EBU teletext protocol for the VBI of a TV signal can be used to transmit subtitles, this is widely used in Europe for heard of hearing subtitles and also for multi-lingual subtitling.
DVB Subtitling is used in digital DVB transmissions. Since it is bitmap based, it supports any language and is therefore not only used in Europe, but also commonly in Asia and other parts of the world. Line-21 closed captioning is also using the VBI in TV signals to transmit subtitles. This standard is mainly used in Unites States for heard of hearing people.
Read more about subtitle file formats and subtitle transmission formats.