Just when we thought iTunes had convinced us to pay for our jams, it looks like we've come full circle, back to free. Today, streaming music service MOG will join other services like Spotify and Pandora, offering a "gratis" option, reports The New York Times's Ben Sisario. "MOG, an American music streaming service, has developed a free version to compete with Spotify." But is the version really free? Just because you don't pay out of pocket, doesn't mean the service doesn't have a catch. As with all neo-music listening options, it elicits subtler payments. No free streaming music service out there comes without a cost.

First and foremost, you pay for your free streaming music with ads. MOG will join Spotify and Pandora as "free" streaming music services that sprinkle advertising throughout your listening, explains IT World's Peter Smith. "FreePlay is also an ad-supported free music-streaming service that gives listeners access to all 11 million songs in MOG's library." Every so often you get to hear a pitch for some product advertisers hope you'll purchase. You can tell yourself you'll never succumb, and maybe you won't, but at the very least you pay in time and annoyance factor.

And MOG and Spotify know you hate ads. So, they're use this tactic, hoping to convert you to a paid subscriber. These services don't want to give away content, explains AllThingsD's Peter Kafka. "Ultimately Rdio, MOG and Spotify, along with services like Rhapsody, are all trying to get to the same place. They want customers to pay them a $10 monthly fee for unlimited music that works on both the web and devices like Apple’s iPhone and Google’s Android." And they each have their own strategies for how to nag you into paying. Spotify is only offering unlimited "free" streaming for the first six months, after that it caps listening at 10 hours/month. (That's only one work day's worth!) MOG has a similar scheme, "sweetening the deal a bit by making the service ad free for the first 60 days," adds Smith. But even before the ads kick in, MOG reminds you that the paid option has less barriers to listening. FreePlay relies on a 'credit system' that depletes as you listen to music. You can earn credits by sharing music, referring friends, listening to new bands and eventually watching trailers. Doesn't that sound like fun? And of course, all the while you're doing this, MOG serves more ads.

Of course, free music, even with ads, is something we've become accustomed to and accept, but this service might not even be worth it. There are lots of other options--Spotify, Rdio--MOG needs to provide something worthy, argues Endgadget's Zach Honig. "Now that we have several free streaming options, it ultimately comes down to availability -- having access to 11 million tracks could be great, but only if you can play the songs you want to listen to." But even if you like the offerings, you can't listen to it on your mobile device for free. Like your Econ 101 teacher always said: They're no such thing as a free jam.