Online artist registries are ubiquitous nowadays. In lieu of or in addition to a personal web site, they are a great way to make your work as an artist available to the larger world. Some are open to all, while others are curated by discipline, geographic region, quality of work, etc. I participate in several registries as an artist, but my hands-down favorite is the Irving Sandler Artists File offered by Artists Space in NYC. Developed by artist/former staffer Letha Wilson and her colleagues, the registry went online a couple of years ago with great success. It is free, uncurated, and still fulfills its primary mission marvelously: to showcase artists, their work, and ideas. Plus, the user-friendly search interface enables you tag selected artists and add them to your online portfolio for further review.
Other registries, however, are far less successful. A perfect example of what not to do is Saatchi Online. It is poorly designed, an aesthetic disaster really, and attempts to do too many things for too many people. I question if this registry is even really for/about the artist community. Ambition can be good thing, but never mission creep.
Artists: Be selective about the context in which you show your work. Context always has a profound impact on how your work is perceived and understood. Weigh the pros and cons, and choose wisely.