I have found this question on yahoo! answers. The question of course is load with ambiguity. What is free on deviantart? The user account? Or the images and artworks in it?
The basic account of deviantart is free. But you can upgrade your account (like mine), to a subscribed membership and pay some small amount of money to get upgraded facility. You can have more advanced features of deviantart using this subscribed membership. Some of the features is banner free (makes faster loading), thumbnailed deviation preview (this feature is really helpful), custom css on your deviantart journal (to impress the girls), and I suppose many more.
What about the artworks then? Well on default settings, the artworks is protected with common use license. I don’t know what the hell would that means. But If I am guessing right, it could means that maybe the creator of that artwork wouldn’t mind if you use their artwork for desktop background or have fun with it but don’t get it wrong. You can’t just upload them to your website and claim it as your own. This happens a lot and making the real artist real concern. This is called the art ripping.
I join the deviantart since november 2006, met a lot of great people there.

