Web 2.0 are blogging platform which allows you to posting blogs, allow to change your URL, Images and also in some websites you can put videos. This one is very helpful for link building.
These sites built your Page Authority very fast.
Web 2.0 is a gold for a OffPage SEO because it permits you to built links and from these links you can indexing and ranking your web-page and also website from that.
You can found famous and good web 2.0 sites from the following list.
http://www.merchantcircle.com/
http://www.homebasedbusinessprogram.com/