A new project just came over the horizon: crypto.cat
One of it's more noteworthy features is that it runs right in the browser, for example here. It is also open source, so theoretically one could run one's own crypt.cat server.
And I quote:
"The IM proxy is the best I’ve seen. Once it’s enabled, every incoming and outgoing IM conversation is logged. After opening up a few channels in IRC - in real-time - it’s possible to view any conversation going through the firewall. MSN, AIM, and other protocols are supported as well. It’s a big-brother feature, but if you want to monitor who you children are talking to, or for whatever reason, I can see it being an invaluable resource to monitor what is going on in a network you control. It would almost be easier to keep track of conversations using the logging tool in Smoothwall instead of multiple instant messenger clients."
The above quote is from a review of the free version of the Smoothwall firewall -- you do not even have to pay money for this feature in Smoothwall! Meaning this feature is probably simple and common among commercial firewalls. Anyone who thinks that their employer is not listening in on their Yahoo Messenger / MSN / AIM / etc. chat sessions is being extremely naive. Check out this:
"New IM Reports - Generate new reports on IM including time spent messaging and number of chat friends per user."
The solution: use a chat client that does encryption: