Nowadays, you can judge the cultural impact of a song by the volume of fan covers that hit the Web. Some of these covers are great, some are not-so-great-but-so-adorable-that-we're-willing-to-forgive-them, and some are just plain terrible, like, so terrible that we wished the original artist had never released it, and we were saved the pain of the cover. Here's a selection of the best and worst fan covers.