One of the most effective ways of getting exposure for your music without spending a penny is to reach out to music blogs. Music bloggers already have an audience that you can tap right into if you're music is featured on their site. Contacting music blogs that regularly feature your genre of music is a win/win for both parties. The bloggers get content to post to their blogs, and you get free exposure for your music.
Here's a list of music blogs by genre: