Sociology
The study of culture and how it affects people and people affect it. Sociology is a great class in general, but for writers specifically it helps shatter your ideas of "normal" and how a culture "should be". It also breaks down a concept as enormous as culture into smaller blocks that are a little easier to work with.
Psychology
Get a better understanding of how humans/sapient beings tick.
Abnormal psychology - which most intro to psych courses at least touch on - will help anyone looking to write mental illness with more realism.
Bonus points if you have to write any sort of scientific journal articles for these classes; go through a few psychology classes and you'll never have trouble writing excerpts from textbooks or scientific documents in your stories again.
Biology
A bit more for sci-fi writers or those interested in non-human characters, but it's still an intriguing look at how life as a biosphere works. Building cohesive ecosystems is easier when you understand what a cohesive ecosystem is.
Anthropology
Cultural anthropology is especially helpful. It's something of a sister class to sociology, peeling back the veneer of social norms a bit. It's also useful for learning about how cultures and parts of culture develop among different groups.
History
How have economics, culture change, revolutions in technology, war, environmental disasters, and much, much more, affected humans in the past? They say the past is the best indicator of the future; if you're looking for human behavior, it's hard to get more accurate than the huge test chamber that is human-inhabited earth.
This incredibly short post brought to you by the huge Software Engineering project I have due on Monday, and by the letter P. P stands for such wonderful words as "panic", "pressure", "pain", and "please please please let this be done on time".
