You could use Candy (http://candy-chat.github.com/candy) as a nice web UI for XMPP MUC (Multi-User-Chat). It supports basic features but is easily extendable through plugins.
Apologies for not being clear. We all already have and use Google Chat. Failing that there are XMPP clients on all the relevant desktop and mobile platforms.
The one thing that has the lowest utility is a web based interface to chat since it essentially doesn't work well in the background in the same way as an app like Pidgin does.
You could use Candy (http://candy-chat.github.com/candy) as a nice web UI for XMPP MUC (Multi-User-Chat). It supports basic features but is easily extendable through plugins.