The online music service, which started making noise a year ago when it announced a licensing deal with Universal Music Group, finally opened its doors September 17th, offering free music to the masses.

That’s right. Free. As in no charge, no bill, no nothing. When it comes to snagging tunes at SpiralFrog, time is the coin of the realm.

Specifically, your time. As in spending your time watching advertisements.

SpiralFrog acts pretty much like subscription services, where a flat monthly fee gives you all the music and video downloads you can suck through a broadband connection. Cancel the subscription and the content stops playing.

But at SpiralFrog you’re not spending any money. Instead, you watch an ad, then download a song. Checking in with SpiralFrog at least once per month to watch more ads keeps your downloads rocking.

Also like subscription services, SpiralFrog songs can be transferred to any portable device branded with the “Plays For Sure” logo, such as most devices playing songs protected by Microsoft’s Windows Media digital rights management technology. However, noticeably absent from devices that “play for sure” is Apple’s iPod, the biggest-selling personal music player, and, ironically, Microsoft’s Zune, which employs a special, custom flavor of the Microsoft DRM.

Another characteristic inherent with subscription services is the lack of any CD-burning capabilities. While portable digital players must be synced to the computer to determine if your subscription is still active, CD players have no such capability. Hence, you can transfer as much as you want, but burning gets you nowhere.

At launch, the service offered 800,000 music tracks and 3,500 video files for download, with much of the content coming from Universal, the only major label to sign on.

The concept of giving music away in exchange for consumers willingly exposing themselves to ad messages has been kicked around since the original Napster willingly facilitated the first part of that equation – songs for free. Now it’s time to see if people are willing to balance that off by parting with their free time.

“We believe it will be a very powerful alternative to the pirate sites,” SpiralFrog chairman and founder Joe Mohen said. “With SpiralFrog you know what you’re getting … there’s no threat of viruses, adware or spyware.”