Audiggle is an interesting tool that can identify many songs from their sound alone.
The program can accept input from any PC audio source. Point Audiggle at your microphone, say, and you can have it try to identify a song playing on your TV. Or, if you choose the "What U Hear" option, then the program will analyse whatever is playing through your speakers at the moment: your media player, internet radio, background music in a game, whatever it might be.
Does it work? Our initial tests were generally positive, with Audiggle correctly recognising a lengthy list of old classics (and provided links to their downloads on Amazon and iTunes). It misidentified an obscure remix, and couldn't recognise a couple of new and relatively minor tracks, but that was no surprise. It's certainly worth a try, as long as you don't mind having to create an account and provide a little personal information (email, gender, birth date, country).
Please note, on its first launch we found Audiggle took a very long time to appear (a couple minutes). This might have been some condition specific to our test PC, but if you find the same, be patient, it does pop up eventually.