It’s hard to be original in this day and age, but some artists frequently cross the line from "being inspired by another musician" to "blatantly ripping them off." And it happens all the time — you probably just haven't noticed.
Although many of these artists get slapped with lawsuits, due to the finicky nature of copyright law, most cases are thrown out or settled out of court. At the end of the day, the best way to judge certain songs and determine if they were in fact plagiarized is to listen to the similarities yourself. Although there are tons of songs that have been accused of copying, the following tracks are, I think, the most obvious. But beware — once you hear them, you might think differently about a few of your faves.