There are several high traffic websites that allow artists to upload work and maintain a free portfolio. As an artist, it's in your best interest to be involved. Remember to keep your work and information updated, and always include a link back to your website.
The UK based Saatchi Gallery launched an online community for artists called Saatchi Art in 2010 that has become very popular among artists and collectors all over the world. They also hold "showdowns" where artists can submit work to compete against each other, and the winning artworks are displayed at the actual gallery. You can arrange to sell your original work (and giclee prints if you have hi res files) directly through Saatchi, and they take a 30% commission. The artist is responsible for packaging their art, and Saatchi pays for shipping and schedules a pickup from your studio.
Absolute Arts is affiliated with World Wide Arts Resources and has become one of the leading websites for artist portfolios since 2005. They offer a free account, as well as a premiere account for $50 per year which gets you more features including more exposure. You can arrange to sell your work directly through Absolute Arts, and they take a commission.
Etsy started out as a small company in Brooklyn in 2005 as an alternative to eBay with the focus on buying and selling fine art and handmade goods. Today, it has 1.7 million active sellers and 26.1 million active buyers. Listing an item costs $.20, and there is a 3.5% fee on your asking price when the item sells. You won't find the serious art collectors spending $10,000 on a painting like Saatchi, but I do recommend keeping active portfolio of art for sale $1000 or less.
Carbonmade seems more geared towards the illustration and design community, but I'm including it because of its clean, simple approach and sensibility. Take the time to create a basic five image portfolio and join the gang. The website was put together by a group of NYC designers in 2005 and shows no signs of slowing down.
Deviant Art, based out of Hollywood, CA, has been online since 2000 and provides a free platform for any artist to exhibit and discuss art. The design isn't as clean as the others, but there is a lot of content and it ranks well with the search engines.