Pandora is a personalized music streaming service that offers a free account to be used on any device whether a laptop PC or your cell phone. The free version includes ads, but you may create custom radio stations and discover new music streaming waves.
Although you can use the Pandora service without signing up for a free account, but you cannot create customized stations and return to them later without registering.
Free Pandora is ad-supported service where you can generate station of your choice and listen to it.