2 neat sites by the same guy; Jeremie of jabber fame:

gibeo.net
This is a proxy that allows users to leave comments about the proxied site and read comments left by other users.

You do this by appending the domain you want to access with gibeo as a subdomain to gibeo.net. Like so:
http://blog.atariboy.com.gibeo.net

Social web commenting / graffiti! Very neat

on2me.com/
This one allows chatting directly on a webpage. There are others around but this looks easier and nicer than ones I have seen.

Would be nice if this would work on any site with the domain append method of gibeo which could add a floating CSS window. Perhaps in the future.