Over the last 4 years, ATLAS has taught hundreds of TOEFL iBT students. After each student takes the test, we do an exit interview and ask them about their experience.
We usually record the topics for question 2 of the Writing section. These topics fall into 6 big categories or 30 subcategories. These are represented in the area graph. The bigger the square, the more common the topic. You can click on any of the topics to download our material for that topic (or read a list of topics here).
If you are taking your TOEFL test soon and only have a little time, I recommend starting with the big squares. However, if you have lots of time, work on items that have the same color. For example, do all the red squares, then all the green squares, etc.
In terms of big topics, the most popular is "Personal" which involves giving advice about personal relationships, describing people, and explaining why people do things. While these are simple in terms of vocabulary, many students have trouble thinking of reasons to support their opinion.
The next largest section is "Education," which involved schools, teachers, students, and learning. In my opinion, this is also the most useful topic for TOEFL students because there are lots of Listening and Integrated questions that deal with similar topics.
Many TOEFL students focus on topics like Business or Companies. In fact, these topics are relatively rare in the Writing section (and very rare in the Speaking section).