Drag your libraries. Image source: MashduoMine was originally a mixed marriage: I'm a Mac, my wife was a PC. Years ago, though, after yet another virus had rendered my beloved's Windows machine unusable, I insisted she switch. (She did so begrudgingly, but she's since become a contented Mac user.) I smoothed the transition by copying all of her old files from her Windows PC to her Mac, but some tracks from her iTunes library, for whatever reason, didn't made the leap.
At the time, we didn't bother to figure out which tracks were missing, but for my wife's birthday this year, I decided I'd finally find those tracks and bring them over to her Mac. I'd assumed it would be a painstaking process: I'd need to look for a couple hundred songs -- out of thousands -- that existed on the old PC but not her Mac. And, of course, her library has grown substantially since the switch, so comparing the two libraries would be far from simple.