As web-based software and website hosting becomes less expensive and more user-friendly, many websites are offering free space for artists to display their artwork. Some of these websites also offer to act as a liaison for sales, to varying degrees. There are a variety of free and paid services, and the quality of these websites and services also varies greatly.

For the most part, these services are legitimate. As with any service, there are a few which are unscrupulous and blatantly resell your images and/or contact information without your permission. Of course, some are less blatant and simply hide that information in a very long list of Terms & Conditions.

Some things to consider before enrolling in an online gallery:

Price
The cost of a domain name and hosting for a small website can cost approx $50USD a year. Although this may seem like a high amount at first, you will want to balance that against the services you are getting in exchange for a small gallery page on someone else's website.

Quality & Professionalism
By the very nature of these services, your artwork will be displayed alongside the work of many other artists. This has advantages and disadvantages; the most prominent disadvantage being a cluttered art-viewing experience which can easily overwhelm visitors and make your artwork seem less valuable.If you decide to make use of these services, ensure you join a website which offers a clean interface (so the website doesn't distract from your art)
Few of these services screen for quality of art, and equally few spend much (if any) effort to promote and sell your artwork. In the end, many of these websites simply fall victim to scammers who use them to target emerging artists.

Rights you retain and rights you give away
Many of these services have a long list of Terms & Conditions in which users have agreed to give the company some or all rights to the use of their artwork. Although, at best, this is simply to allow the service to reproduce your artwork on its website, this usually legally allows them to do anything with your artwork---including reselling it without a commission to you. This is typical of websites where the content is provided by users (you), but it can also be detrimental to an artist's professional career and sources of income.
For more information, also see Real Websites with Unfavourable Terms and the post on Saatchi Gallery - "Your Gallery".

In the end, there is no such thing as a free lunch. If you are serious about having an online presence for you and your artwork, you are strongly encouraged to spend your time & money on a personal website so that your investments and rights are protected.