I had a Psychology professor that would make 4 (or 5) different versions of an exam for his 100 level courses. They weren't different questions, they were just arranged in a completely different way. Some people would start with essays, others would start with short answer, etc . . . In addition to this, he had arranged a seating chart that made it difficult for two people with the same exam to see each other. He forbade hats and cellphones and had his TAs roam around looking for cheaters. Very impressive, though it did take about 20 minutes to get everything ready for the actual exam.
And then I had the Art History professor that accidentally left some of the names on the art slides we were supposed to identify. When a TA attempted to point this out, the professor didn't seem to understand the problem, until the TA literally had him look at the screen and the test at the same time. He was a good sport about it though, he laughed for a solid minute.
I witnessed a lot of cheating my Freshmen and Sophomore year, but it ended almost entirely after that point. But, my major was History and after sophomore year most of my courses were based around research papers or discussion sessions. I don't know if this was simply to hone our research skills or to also curb plagiarism, but my professors made us pick research topics that weren't well covered. Some of these topics were so far out I think you'd spend more time trying to find a paper to plagiarize than actually doing the research.
As said, though, at the 100-200 level I saw a lot of cheating by my fellow students. Copying from other people exams was the most prominent, I think. The way lecture halls are set up, its easy for people to see the exams of folks in front of them. Also, when there are a lot of students in a class, it's hard to notice people that are using notes while taking the exam. Plus, the wide range of topics in most 100-200 courses lend themselves to multiple choice exams, which are much easier to cheat with than essays or short answer. I doubt 50% of students cheat, but I wouldn't be surprised if 1 in 4 students cheated at some point during their college education.