Second Life is a virtual online world, or metaverse -- a term popularized by Neal Stephenson's 1992 novel "Snow Crash" and used to describe three-dimensional virtual environments like Second Life.

While it mirrors the basic functions of social networking sites such as MySpace and Facebook, Second Life (secondlife.com) offers a more engrossing experience. Like other social sites, users can meet people and form special interest groups and communities. But with its intricate 3-D environments, it's a frontier that some users believe points the way to the next-generation Web experience.

There are three levels of membership. With the basic level, which is free, one can create one avatar and build things in free temporary building areas. To create additional characters, there's a one-time $9.95 fee. For $9.95 a month, residents can own land and build things on it.

Second Life was launched in 2003 by Linden Research Inc., also known as Linden Lab. In recent years, it has become a more mainstream phenomenon.

It evolved out of popular role-playing online games that have been around since people first started exploring the Internet.

Second Life isn't just about play. Its economy and cult following has attracted the interest of real-world businesses and media. A recent TV episode of "CSI: NY" incorporated Second Life graphics into its plot, and CNN has launched a citizen journalist project within the site. Coldwell Banker recently set up a virtual real estate sales office there, hoping to attract young buyers in the market for real property. And Kelly Services Inc. launched a virtual headquarters in Second Life last month, marking the occasion with a live blues concert.

But even as Second Life moves closer to mainstream culture, critics have noted its limits. A recent study by technology research and consulting firm Yankee Group says the need to be plugged into a computer hinders SL in terms of widespread mainstream use. The report indicated that average use per unique SL resident declined by 66 percent in the past year to 3.8 hours per month, and that less than 1 percent of the population uses the site on a regular basis.

"To achieve greater potential in the marketplace and grow beyond early adopters, the experience must be untethered," says Yankee Group senior analyst Christopher Collins. "Companies that provide remote access ... will have a greater impact than PC-centric companies."

Other virtual worlds to explore include there.com, which has its own economy and where residents get around with skateboards and airborne motorcycles. Virtualibiza.com is a cross between a virtual world and an online dating service, where members can hang out and meet other singles.

An alternative to SL is the porn-free, kid-friendly Meet-me out of Japan and scheduled to launch next month.

There's even a parody site -- Get A First Life -- designed to get people more involved in their real lives.

For more sites and info, visit www.virtualworldsreview.com.