Spotify is testing a US-only beta program that allows musicians (or their teams/labels) to upload music directly. It's an add-on to the existing Spotify for Artists service. The requirements are here. Allegedly this will be "free to all artists, and Spotify doesn’t charge you any fees or commissions no matter how frequently you release music."
You'll need a Spotify subscription, and it's unclear to me whether or not it needs to be a premium (paid) version. If so, it's a nice perk to add to an already $120 USD annual subscription.
I can't imagine it'd be completely free. It still costs Spotify money to store, stream and otherwise manage all that digital music.
No idea yet on what happens to your music if you cancel your subscription, though! When I was learning to write iOS apps, I had to pay Apple $99 USD per year for the privilege of keeping my apps in the App Store. Suffice to say, you can no longer find my Metafilter Radio app there. Does something similar happen to your music if you let your Spotify subscription lapse?
Who knows? I doubt most indie artists (read: no promotional budget (or desire)) will make enough from teeny-tiny streaming royalties to cover a paid annual Spotify subscription. That said, streaming royalties aren't the real indie game -- it's access to the music-listening world that counts.