That's where popular app Shazam comes in.
Shazam has been around for awhile but it's pretty useful.
Open the app when you hear a song, press the button, and voila! Shazam tells you what song you're listening to, and then saves it for you so you can find it later.
Recently, the app put together a list of the Shazam "Hall of Fame" - the songs app users were searching for the most. The "legend" songs were searched for over 15 million times each, the "superstar" songs had over 10 million searches, and the "stars" had over 5 million.
Here are the top 25. The Shazam search numbers are as of the time of this post.