SoundCloud is a great tool for sharing your music with others if you're in the music business, and thanks to that, it's also an excellent community for finding new, free tracks for your listening pleasure.
Once you sign up, you'll be encouraged to upload your own tracks and send them around. This is very quick and easy to do if you've got your own music (that you wrote) to share. If not, you can start browsing.
From the "Tracks" drop-down menu you can "Explore Tracks" and see the music that's popular at the moment. Groups will help you get a little more specific when you're looking for a particular track. Search will let you, well, search for exactly what you're looking for. Nothing too new here aside from some really great music you've probably never heard before. One browsing option that's pretty nice is Creative Commons-licensed music. You can browse through music that's often free for you to use however you'd like.
If you really want to like a song and want to share it, SoundCloud provides a nice embed player. You can also comment on specific moments in a track to let the creator know about a moment you really liked. You can also follow that particular artist to receive updates when they add new tracks to their SoundCloud account.
SoundCloud is free to use and really refreshing if you're looking to discover new music outside of the limited pool you'll find on most music streaming sites.