Open letters usually take the form of a letter addressed to an individual but provided to the public through newspapers and other media, such as a letter to the editor or blog. Especially common are critical open letters addressed to political leaders.
Currently there are very few sites solely specialising in publishing open letters. However, there are community sites where visitors can publish their own letters and promote them to a wider audience. 
Letters patent are another form of open letter in which a legal document is both mailed to a person by the government, and publicized so that all are made aware of it. Open letters can also be addressed directly to a group rather than any individual.
Motivations for writing open letters
There are a number of reasons why an individual would choose the form of an open letter, including the following reasons:
- As a last resort to ask the public to judge the letter's recipient or others involved, often but not always, in a critical light
- To state the author's position on a particular issue
- As an attempt to start or end a wider dialogue around an issue
- As an attempt to focus broad attention on the letter's recipient, prompting them to some action
- For humor value
- Simply to make public a communication that must take place as a letter for reasons of formality
- Many of the epistles of the Bible (such as the Pauline epistles) are open letters.
- Encyclicals are by definition open letters sent by the Pope (in the Catholic Church) or a primate (in the Anglican Communion) to the bishops of the church community but also published for general consumption.
- Most papal bulls are letters patent and therefore open letters.
- Martin Luther published many open letters, including his Open Letter on the Harsh Book Against the Peasants
- Farmer's letters by Samuel Seabury against the American Revolution
- William Banting's Letter on Corpulence (1863).
- Professor James Syme's calls for medical reform in 1854 and 1857, addressed to British Prime Minister Lord Palmerston.
- Robert Louis Stevenson's open letter to Rev. Dr. Hyde in defense of Father Damien.
- J'accuse (1898) by Émile Zola over the Dreyfus Affair
- Martin Luther King's Letter from Birmingham Jail (1963)
- Bill Gates's Open Letter to Hobbyists (1976) attacking copyright infringement in software development
- David Cross's open letter to Larry the Cable Guy.
- The open letter given to Tarja Turunen by the other members of Nightwish, firing her from the band at the climax of End of an Era. She subsequently responded with her own open letter in reply.
- Bobby Henderson's Open Letter to the Kansas School Board (2005).
- Google's Open Letter to the net on net neutrality 
- Steve Jobs's Thoughts on Music (2007) concerning the past and future of DRM.
- Siegfried Sassoon's A Soldier’s Declaration, questioning the judgment of Britain's leadership in World War I.
- Sam Harris' book Letter to a Christian Nation is written as an open letter in response to criticism he received after his previous book, The End of Faith.
- Rob Lewis' Entrepreneur's letter to The Times warning of the dangers of a Lib-Lab coalition ahead of the 2010 UK general election.
- Tom Wrigglesworth's Open Letters, a BBC Radio 4 stand-up comedy series taking the form of an open letter. Based on Wrigglesworth's 2009 Edinburgh Comedy Award nominated show Tom Wrigglesworth's Open Return Letter to Richard Branson.
- Merriam-Webster's Online Dictionary
- Macmillan Online Dictionary
- Opnlttr community site for people to publish open letters
- Open Letter to Larry the Cable Guy
- Open Letter to the Kansas School Board
- Google.com website on Net Neutrality
- The Times: 29 April 2010: Lib-Lab coalition would be 'disastrous for British business'