Check out the Song Tapper at Bored.com. Using this tool, you can tap out the beat of any part of the song you're thinking of, and the tool will try to match your beat to a song in its records. This is particularly useful if you can't specifically remember the sound of a song, but remember the beat.
Go to Midomi.com and use their music search tool by selecting "Click and Sing or Hum." You will be given the opportunity to sing the song out loud and the search tool will compare your recording with its own library of music. The Midomi music library is very large compared other similar tools, and therefore presents a greater chance of identifying your target song.
Download the free version of the "Shazam" application if you use an iPhone. Whenever you hear the song on the radio or out-and-about somewhere, load up the "Shazam" app, and click "Touch to Shazam." The app will listen to the song playing and analyze the incoming recording. Once the song appears on the screen, "Shazam" will present you with the option to buy an MP3 of the song as well.