There are themes for IELTS letters (writing task 1). I assembled it from different sources. If I find a new one, I will add it here.

Information about IELTS letter

1. No more than 20 mins on letter.
2. You should write over 150 words.
3. You will be market on:
   1. Task response
   2. Coherence & Cohesion
   3. Vocabulary
   4. Grammar

Common IELTS letter themes (ask, describe that the question is about, give additional requirements and explanations):

• not attending to the birthday party
• a large company in your area has decided to spend a certain amount of money
• ask HR manager about no response after interview
• ask translation agency about the report to be translated
• asking for information regarding the contents of the exhibition
• complaining about some adolescents let their dogs run wildly
• complaints about the reception area where visitors to your company arrive
• during a recent plane journey, you sat next to a businessman
• for the past year you have been a member of a local club
• letter from a friend for advice about whether to go to college
• letter to manager asking permission to attend the conference
• letter to the friend who has a plan to visit your city
• letter to the municipal authority complaining about damaged road
• letter to your friend about the museum you have visited
• people are having problems with their internet connection
• problems with the public transport you use
• recently you received a letter from the bank
• to a city council about dirty air in your street
• to institution in Australia where you wish to undertake a short term training (what training, why it is useful, details and time)
• to manager about parking lot for your bicycle
• to newspaper to ask writer credentials
• to the company manager about construction near your home (voice, plans, your hopes)
• to your friends about party (date and location, who is coming, about occasion)
• to your manager about organise a party for a colleague
• while you were driving on holiday, you had a small accident
• why you think the restaurant is worth visiting
• write a letter to an employment agency for a part-time job
• write a letter to the Information Officer at the TAPE Information Office
• write a letter to the person in charge of a venue
• write a letter to the restaurant to thank them
• you are due to start a new job next week
• you are employed full-time and also doing a part-time evening course
• you are going to have a family party in a hotel
• you are going to visit New Zealand for an 'English and Homestay' program
• you are not happy about a service you received
• you are organizing a trip to the Snowy Mountains
• you borrowed an important textbook from a classmate
• you borrowed some books from your school or college library
• you cannot go to a company where you got an offer
• you cannot join the course due to some reasons
• you had a holiday overseas where you stayed with some friends
• you have a friend who lives in a city abroad
• you have bought a new mobile phone and it has stopped working
• you have recently been to stay with an old friend for a few days
• you have recently gone to live in a new city
• you have recently moved into a different house
• you have recently moved to a different house
• you have recently started work in a new company
• you recently attended a meeting at a hotel and left some important papers
• you recently enjoyed a meal at a restaurant to celebrate a special occasion
• you recently had your computer fixed at a local computer store
• you recently took a part-time job working for a local company
• you recently visited a place that had a strong impact on you
• you travelled by a bus and your suitcase was damaged
• you travelled by plane last week and your suitcase was lost
• you were disappointed with the meal and wish to complain
• you will move to a new city for work
• you work at home and have a problem with a piece of equipment
• you work for an international company and want to do a training course
• you would like a friend to accompany you on a holiday
• you would like to move into another room
• you would like to participate in a work-related seminar
• you would like to rent out an extra room to a student
• you would like to suggest a place you have visited lately
• your neighbours have written to complain about noise
• your rented accommodation was recently damaged
• you cannot go to a meeting, so somebody else will go in your place

Sources of information was used for preparing this article:

1. IELTS Liz
2. IELTS Mentor