Chatsum is a browser-extension-for-chat-on-web-pages. My #8 on the list.
Remarkably Xpanity is available for Firefox (as an extension) and IE (as browser
The protocol seems to be XML based.
Not many users. The Xpanity web site has a "Web Map", which claims 35 users on
youtube, but when I went there I found nobody, just old chat lines. They had no
timestamp, so I can not really judge how old. But 35 visitors seems like an
exaggeration. At least nobody answered.