I don't know useful this site is for becoming a well known artist, but its a way to get your art out there and they have occasional contests that have cash rewards and provide an opportunity to get your art seen by thousands.

To me, the most useful part of deviantart is their collection of stock images. There are a ton of them and they are not necessarily easy to sort through but most can be used for commercial or personal use for free, the one who submitted them just wants a heads up. This is great for graphic design or for traditional art.

The also have free tutorials, some of which are quite useful especially for drawing cartoons and anime and for Photoshop techniques.