There are quite a few of these.  Here is a sample.

Second Life is the big player with 20 million users (called residents).  You will need a computer with a decent amount of power, plus a pretty good broadband connection, but the results are stunning. 

It has its own currency, the Linden Dollar, and you can earn money or spend it. You can even get it stolen.  There are people everywhere in this world (well, avatars anyway), and it is HUGE.  This is a very cool experience.  You can buy land and build on it, but you need real dollars for this.  http://www.secondlife.com 

3b is a more web-related experience. There are a number of different locations, and each location is lined with 'posters' which are web pages.  You can enter a web site via these screens when the program becomes a standard web browser.  The locations are fairly small and you won't get lost.  You sadly will not meet too many people either.  However you can set up your own village and invite your friends. http://www.3b.net

Multiverse is really software for hosting virtual worlds.  They have some demonstration worlds set up but that is as far as it goes so far. http://www.multiverse.net/

This is definitely a multi-user computer game.  Millions of players in a fairly typical sword and sourcery type environment.  Fight other players, earn points, go up levels.  The usual stuff but on a massive scale.

Active worlds is a lot like second life.  I like the fact that if you have not signed up you enter as a tourist.  The tourist avatar wears shorts, a hawaiian shirt and a goofy hat.  Nice touch!  It is not as highly functional as second life and I don't know if they are going to get critical mass.  There were quite a few tourists though.

The download and set up is fast and painless.  If you want to sample the concept this could be a good place to start.   http://www.activeworlds.com/

Planet virtual is another virtual world / game I came across, although it looks rather down-market and I wasn't sure about loading software from this site.  http://www.planetvirtuel.com/

There.com

Big download and install. I got bored and cancelled it. Sorry.  http://www.there.com

Whyville

This seems aimed at kids with more of an educational objective.  http://www.whyville.net

Entropia universe

This looks a lot like a game.  We seem to be talking about robots and monsters here.   http://www.entropiauniverse.com

You have a profile, and you behave 'socially', so you have to ask whether this is the next generation of social networks?    You can meet your friends, talk, and it is a much more dynamic and generally cooler experience than Facebook or other social networking sites.  After all - you can fly.

So will the teenagers be flying around in second life rather than facebook next year?

I don't think so.  Facebook is all about who you really are.  The pictures are of you, the words are about you.  You can lie of course, but by and large people believe what they are putting up on the site.

In Second Life you play a ficticious character.  My avatar is a young guy with a beard. You can edit your profile with 'real world' information, but hardly anyone does.  That is not the point.  This is all about fantasy. In fact Second Life has much more in common with World of Warcraft than it does with Facebook.   

Basically Second Life is a game. 

Fairly obviously we can start with is billboard advertising.  On 3b the world is lined with billboards and you can put your web page on them.  There are billboards on Second Life as well. 

I think anything that involves presenting something to many people can benefit.  Training, theatre, concerts could all prosper in a virtual world because a virtual world is better at simulating a group experience.  Millions of Second Life residents at a rock concert could be quite something.

You can even have your own space - BT has an area called 'Area 21' and there are red phone boxes there that you can use to place calls to the outside world at reduced rates.  Apple apparently have a virtual store within second life, although I have never found it.  I have to say I have not spent so much time there. I am not otaku yet.

This month we have an outstanding useability column from Jakob Neilsen.  There is an important insight

• Designers are not users

Designers tend to design for people like themselves.  Did you know that in trials, 25% of users were unable to conduct a search on Google.  That is they either failed to run a search at all or ran on some other search engine, usually from whatever type-in field happened to be on hand.

 http://www.useit.com/alertbox/designer-user-differences.html

My favourite ad server software has a new name - OpenX.  I don't know when this last change of name happened, but it is the last of a long chain.  In the beginning was phpAds.  Then phpAds begat phpAdsNew which begat MaxmediaManager which begat OpenAds which finally begat OpenX.

It is basically the same product although the internals have been rewritten in the past two years - when the developers got any time from thinking up new names.  Hopefully OpenX will be the name now for good. 

Vista SP1 is now available, it will get installed automatically this month but you can download it now from the update centre.  I recommend you do it if you have been suffering with Vista.  SP1 doesn't exactly rock, but it shows a big improvement in performance. 

Silverlight 2 is coming. I still haven't got to grips with what Silverlight really is.  It seems to be a competitor to Flash from Microsoft.  Quote: "To bolster RIA development, the Beta 1 release will include a Windows Presentation Foundation UI framework for building rich Web applications. "  That's all clear then.

IE8 is coming.  This I understand!  And IE8 is going to be really really standards compliant chaps so lets remove all those kludges to get our site working with IE7, and IE6, and IE5....

Paypal is warning Safari users that the browser lacks anti-phishing features and they advise customers not to use it.

Big news - Google has bought DoubleClick, the giant online ad company.  At $3.1 billion - a snip.   

Google meanwhile will be offering a free hosted ad server for publishers.  This allows publishers to mix in Google ads with their own, which is presumably the motivation behind the offer.   The OpenAds - sorry OpenX blog responded "Google becomes both an ad network, ad technology provider and a competitor to ad revenue to publishers. As a publisher, I would find this a dangerous cocktail and I would worry that it may marginalize my revenue. "  http://www.openx.org

AOL is buying Bebo for $850m.  You probably heard that already.  The site is way too pink for me.

Yahoo is moving its European HQ to Switzerland from London.  

Someone bought a stroller in a German online auction site.  It came with a useful accessory if the child gets too tedious - a loaded pistol!  The seller said it didn't belong to him.