Back in '2000 or so, there were approximately 40 products in that space - I counted because I was working on one. They all required some form of download - and non of them got critical mass.

Odigo Communicator; ICQ Surf; Zadu; Hypernix Gooey; Cahoots GetWebby; and a whole lot more. Also, there was an IE addin called 3rd voice that let you annotate a site for others.

I wonder if now is a better time for this kind of products.