15360 Ready-made 'web 2.0' scripts

I'm looking for ready-made systems that contain web 2.0 features. My definition of web 2.0 would be a site that uses social networks, ajax, tagging or any of such newly developed technology. Example of sites would be flickr, 43things, digg, myspace, youtube, [login to view URL], netvibes etc. Please provide demo. I do not have a clear plan of what I would like to do with them, I'd just like to see what's out there and maybe create a site or 2 to see how it goes, therefore my budget is not very high, unless you have something really unique. So please be reasonable with your bids :).