http://elfwood.lysator.liu.se
Elfwood is non-profit, created by Thomas Abrahamsson, it is one of the bigger web communities.
It is well maintained and very cool to browse through. The level of skill the artists and writers have varies from very amateur to very professional, the place is a gold mine of talent.
It's easy to join and a good place to get input about your creations.
The only drawback is you have to be into either fantasy or scifi.
Update (Monday, July 25, 2005):
Elfwood is still to my knowledge the largest non-profit online gallery. It appears to continue to be a fairly well-meaning community and holds true to Thomas' original concept of providing a free of charge online space for Fantasy artists to congregate and display their artwork. It is true however that Elfwood's popularity appears to be in decline, reasons I perceive for this are; the fairly restrictive framework, server instability that puts the site down for days, site data loss, a rediculous and awkward 'extranet' submission policy, and the growth of younger, sleaker, more professional online art communities with much better user interfaces such as deviantart and epilogue. I dont know if Elfwood will ever have the resources to compete with the profit making equivelants, but I know for a fact that most people don't have the patience to wait on ticket queues for their artwork to be put on display when there are sites either offering much better artwork moderation or instant uploads, and much more opportunity for gratification. I feel elfwood needs a makeover and some policy changes, but I, and a lot of other people, still share a lot of affection for the project and it's community.