There is no clear definition for a web2.0 site, then how do you know whether a particular site is web2.0 or not?. James Britt and Neurogami got the same question and created a website to validate web2.0 sites. If you want to see whether a site is web2.0 go to Web2.0 Validator and enter the URL, it will run some tests and tell you the score for that particular site.

These guys created some rules for a site to be called web2.0, these rules are recommended by users, you too can recommend the rules to include in the validator. For suggesting rules all you have to do is go to del.icio.us, bookmark the web2.0validator URL and in the notes section enter some description along with a rule,

ex) Appears to use AJAX : /prototype.js/

Rules

Currently the validator checks 66 rules to give you the score for a particular website.
I checked Youtube and del.icio.us for curiosity and got the scores of 6 and 13 respectively. This is something strange as i expected a bit high score. To my surprise there are some funny rules which are not related to a web2.0 site thats why these sites are scoring low.

In the end it looks like fun stuff. :))