Tag! You're It...

As I continue to look at all of the new tools and technologies that are cropping up all over the Web, I 'm trying to figure out what exactly makes a Web 2.0 application a Web 2.0 application. Is it user customization, social networking, sharing and modifying information, or something else?

The one common thing that I have seen in most of the Web 2.0 applications that I have looked as is tags - the ability to categorize or group what you are doing so as to make the information produced easier to share and find.

So, is it valid then to say that in order to be a Web 2.0 application, tags must be used? While not all inclusive, I think it's a valid "litmus test", and a way to identify something as being Web 2.0. Here are just a few Web applications that use tags. You can decide if they are Web 2.0 or not:

• GMail (OK - Google calls them labels, but they are tags)
• Del.icio.us
• NetVibes
• Blog software (Blogger, Word Press, etc.)
• Wikis (Wikispaces, pbWiki, etc.) Check out Wikimatrix to compare different wikis
  
• Flickr
• Facebook
• My Space
• Technorati

And I know that there are many, many more... Which ones are your favourites?