Grooveshark is my favorite quick way to listen to a particular song online. With an extensive library, easy sharing and embeddable widgets, it's a powerhouse in the music sharing arena. It's only getting better, too, with the recent addition of a Facebook application and a Wordpress plugin. The Facebook app lets you share songs directly to your wall, and the Wordpress plugin lets you quickly put together a playlist widget for your Wordpress blog.

The Facebook app, in particular, is the easiest way to share music on FB that I've run across so far. In addition to publishing a song to your profile, you can also select individual friends to share it with. With over 10 million songs in Grooveshark's database, it's likely you'll find whatever song you're looking for.