Sometimes you take classes with a goal in mind, and sometimes you take classes just because they're interesting. I've taken non-degree classes at two periods in my life: during high school and in preparation for my medical dosimetry program.
Medical Dosimetry Prerequisites
Since my earlier education was focused on computing and business, I needed to fill in several holes in my experience before going down a path into a clinical education track.
Junior Scholars Program
The Junior Scholars Program was a summer enrichment program that let high school students take college-level classes during the summer between their junior and senior years. These were regular classes taken with regular college students. Alumni of this program could begin their freshman year of college with existing transfer credit.