The New School Continuing Education program is great if you're working towards a degree, or just want to expand your horizons and try something new.
Every semester classes are offered in subjects ranging from computers, to music, to writing to cooking, and the actual classes within these subjects are incredibly varied. Classes can be taken for credit, or just for fun, and the teachers on the whole are FANTASTIC. Tuition ranges from about $250-$500 for a semester depending on the class, with one day workshops of course being less, and classes with special material needs being more.
I took a writing class several years ago with a fantastic teacher, and it was great fun. The only downside was that since registration was open we got students with varying levels of experience, so if you're already in the field it's worth it to look into the advanced classes.
Visit newschool.edu, or call or stop by 66 West 12th street to pick up a class bulletin. Financial aid is available.