Napster boss Chris Gorog launches the service
Napster has launched the world's largest online music store without digital rights management (DRM).
More than six million tracks will be sold as MP3 files, from all of the major record labels and independents.
The songs will play on any MP3 player, including iPods, and can be burnt to CD and transferred to other devices.
Napster boss Chris Gorog said the service, which offers songs for 99 cents (79p), moved online music "from under the DRM cloud".
The Napster store will compete directly with a similar service from Amazon, the only other online store with MP3s from all the major labels, and Apple's iTunes, which offers a limited number of DRM-free tracks.
The move also shifts Napster away from its previous "all you can eat" music service, which let users access as much music as they wanted for a monthly subscription.
"We believe ultimately that consumers will be moving to an unlimited music model," said Mr Gorog, adding that Napster will continue to support its subscription service.
Apple's iTunes remains the dominant music service, with 70% of the US market, and a position as the biggest music retailer in the country.
The arrival of Napster's music store also cements the music industry's shift away from support for digital locks on tracks.
"It's great that we have finally gotten here," said Mr Gorog.
"It is really the beginning of a level playing field, which I think is essential for Napster, but also for the health of the digital music business in general."
Jupiter analyst Mark Mulligan said Napster's new service was in danger of making its own proposition worse.
"This almost weakens their subscription system. As radical as it sounds Napster needs to find a way to offer its 'all you can eat' service and be DRM free."
He added: "That would give it the clear blue water it needs between itself and forthcoming services like Nokia's Comes With Music."