"Music Unlimited powered by Qriocity" may be one of the longest, most obnoxious names for a service ever, but at least you'll get to try it for free if you own a PS3. Sony is providing a free 180 day trial of basic Music Unlimited services to PS3 owners that live in America. So long as you've never tried Music Unlimited, you'll be able to automatically start your trial by simply launching the app from the "Music" section of the XMB. Once the program installs and launches, you'll get a message telling you that your 180 day subscription has begun.
But what is Music Unlimited, and why does Sony think you'll want a subscription? Essentially, it's Sony's attempt at a cloud music service, combining features of Pandora, Zune Pass, and Amazon Cloud Drive. The trial, which normally costs $3.99 a month, will give you access to a variety of ad-free radio channels, with unlimited skipping of songs. You'll also be able to transfer songs and playlists from your PC to the cloud.
Once you activate the service, you'll be able to use Music Unlimited on a number of other devices, including the PSP, Android phones, and your PC. Note that your trial is only for the Basic service, and omits many of the really cool features offered by the Premium service, such as the ability to play any song in the Qriocity library at any time. Also, while your free subscription is not supposed to renew automatically, you should double-check your "Automatic Funding" settings, either from your PS3's "Account Management" menu, or online.