Can't get a song out of your head and don't know what it's called? There are two good applications for iPhone that will fetch song titles and artists when curiosity begins to gnaw.
Shazam for iPhone (see video) grabs song, title, and album information after 30 seconds of listening to a recording. If YouTube videos are available, you'll be able to download them in a click from the results page. When or if you're ready to buy, the application also integrates with the iTunes music store, which will gladly bill your account for the song.
Broader still is Midomi for iPhone (coverage), which supplies singing, hummimg, speaking, and typing as input options in addition to recording a 10 second song clip. Either way you phrase your request, Midomi will return similar artist details, video, and iTunes lookup results.