Courtesy: Ghita of Technical Writers Group One is BitwiseIM (http://www.bitwiseim.com/) which should be pretty reliable over any connection and has also encryption of the messages sent, but that too would require signing-up.
Another is dimdim (www.dimdim.com), a free live meeting media used through the browser. I have yet not that much studied it, but it seemed pretty simple to use and would have all the necessary tools we could think of to be useful at some point. Here is more about it http://www.dimdim.com/aboutus/Who_is_dimdim.html
