Free legal music downloads with a twist. New York-based, SpiralFrog said on Monday it has launched its free music ad-supported Website. After two months of beta testing, the music download startup promises to change the way music lovers download popular songs from major entertainment labels free of charge.

The twist, in return for that copy protected free music to your ears, SpiralFrog says that in order to download its free music, registered users will have to wait for about 90 seconds to view its relevant, targeted advertising and answer few short surveys before downloading.

SpiralFrog’s service allows music lovers to download unlimited free music from its website, giving its users access to a library of 800,000 songs and 3,500 music video catalogs.

Oh, those sweet bytes of free music downloads, the company said that music lovers can easily access music by major acts like Beck, The Killers, Maroon 5, Amy Winehouse, U2, Black-Eyed Peas, Gwen Stefani, Kanye West, Nine Inch Nails, All-American Rejects and 50 Cent, and also discover up-and-coming acts like The Hold Steady, Kaiser Chiefs, Colbie Caillat, M.I.A., Snow Patrol, Klaxons, Marié Digby, Shiny Toy Guns, Art Brut, Aesop Rock, Feist and Sufjan Stevens.

In addition to the free music downloads, users will be able to view millions of artist bios, reviews, discographies and album art, among other content features. SpiralFrog’s registered users will also be able to download its free application to keep track of their downloaded songs on their Facebook profile page.

Like other subscription-based music download website, SpiralFrog requires that registered users must renew their free subscription every 30 days to keep downloading music, and every 60 days to keep listening to files. However, the audio and video files downloaded from SpiralFrog never expire.

While the music downloads from its website doesn’t allow for CD burning or be played on Apple’s operating system, and iPod, SpiralFrog’s songs and videos downloads will only play on Windows PCs running XP and Vista. Users can also download to their PCs and synchronize their music with up to two portable music players or music-enabled phones that support Windows Media DRM.