Mr. Presnell is one of many students across the country using free college lecture videos as a new kind of study aid. The lectures are livelier than textbooks. They provide the sense of a human touch, though they lack the interactivity of a tutor. But mainly they’re free and available 24 hours a day. Some students say they prefer the free videotaped lectures to the live lectures they are paying for at their own institutions. Others say they use the online talks to focus on topics they didn’t quite get when they first heard them in their own courses. And some high-school students use them to get a jump on material they will encounter when they get to college.