Hardly two days have passed since Spotify began allowing unlimited access to its free streaming service, but another major competitor has already stepped up to the plate with a major transition of its own. Rdio, one of the last bastions for premium desktop music services without ad-supported streaming, has finally gone free. Rdio announced on its blog that it has “added in-stream messaging to Rdio on the web,” short ads for free users who don’t want to subscribe. Rdio will still maintain its $9.99 per month subscription service, dubbed Rdio Unlimited, for users who want to avoid the ads. Now that all the major streaming services have settled into a comfortable equilibrium, the impending launch of the subcription-only Beats Music is going to be even more interesting.