You should get yourself on every one of the following sites. The ones that allow you to give away mp3's, do so without you having to give up any of your rights to your music. All of them are either social networks, encourage sharing to social networks, or make it very easy for bloggers to share your music with their readers.
Inform me if you get on any of the sites in bold, and I'll help out as much as I can with spreading the word and getting people to listen, through various means. (G+, Facebook, Friendfeed, and my music blog, if it's an instrumental)
last.fm (you can give away mp3's here)
myspace.comsoundcloud.com (you can give away mp3's here)
reverbnation.combandcamp.com (you can give away mp3's here)
A new online radio site that encourages and rewards listeners for helping out indy artists by promoting them to their friends. You can get mailing list signups through this, too. Then you can offer your free mp3's to your mailing list subscribers.
earbits.comGet on youtube, even if it is just with your music playing as background sound to a static image. Include a link to your site and offer for free mp3's with each video you post.
Sites that could be good or maybe to stay away from if you are opposed to CC licensing:
jamendo.com (they
require CC licensing for you to upload your music)
Get in touch with these guys if any of your music falls into the Instrumental/Experimental/Post-Rock genres:
Lost ChildrenThey go to the ends of the earth to promote and distribute their free CC licensed compilation albums and have a whole section for their releases on
archive.org, and quite a following spread out all over the internet.
And if your stuff is post-rock, specifically, get in touch with the guy that runs the
post-rock page on Facebook and see if he will share your stuff with his Facebook, Last.fm, twitter, youtube (and wherever else he is) subscribers. (he hasn't posted very much music this month because he's been participating in the protests in Turkey, but he will be back to it, as soon as he can)