Grooveshark is a pretty useful web application that allows you to search for and immediately play most songs. Seriously, though, to believe it you have to go to their site and try it out. Not only that but you can bookmark certain songs and revisit the page any time to listen to it, they provide links and a whole slew of other neat features. Some of the features include uploading your own personal music library and creating savable playlists (like iTunes, only you can choose ANY song you want, even if you don't own it!)