Deviant Art is probably the biggest and most well known art community website in existance.
There are many smaller and more specialized art communities. But you will be hard pressed to find one that is as large or as all inclusive. Digital, Pencil, Photography, Calligraphy even writing is considered a form of deviant art at this site; and no site contains as many admirers.
If you are an artist, there is no better opportunity for exposing your work to the public for free than Deviant Art. You can also make friends, chat, request peer critiques of your work and in some cases (see site for details) sell prints (and some other items) of your work directly through the site.
But even more than that, Deviant Art is a community of people which is made up of folks with artistic inclinations. Whether you are a novice or an expert you can find friends, get advice, and immerse yourself in the culture and maybe even gain some fans. If you are an art appreciator there is no other one spot on the web (that I kow of) where you can see such incredible work and have such unfettered access to the artist.
Deviant Art has different accounts, the maost basic of which is entirely free and allows you to utilize most of the functions of the site. If you enjoy art in any form you will most likely find this site quite enjoyable. If there is a specific type of art you appreciate you are very likely, no matter how obscure, to find some prime examples on this site. And if you are an anime of mang fan you will most likely never want to leave.