Apple may start to worry as it’s successful iTunes music store is seriously threatened by the launch of the world’s biggest MP3 download store.

The new web-based music library from Napster comprises over 6 million songs from all major music labels as well as from thousands of independent labels, making the biggest online MP3 store. What makes it diferent from Apple’s iTunes is that the songs will be compatible with the vast majority of digital media devices and mobile phones including Apple’s popular iPod as well as its iPhone. The songs can also be burnt to CD and transferred to other devices.

The move shifts Napster away from its previous “all you can eat” music service, which let the users access as much music as they wanted for a monthly subscription.

Most of the songs on Naspter’s new online store can be purchased for 99 cents, while a whole album will cost $9.95. The Web interface works on Windows, Mac OS, and Linux running Firefox 2.x or Internet Explorer 7.x. The downloadable Napster software, which is required for the service’s portable device syncing, runs only on Windows XP or Vista and requires Windows Media Player 10.