Meebo is going to help you fight back.
Meebo is a web-based instant messaging program (no download or installation necessary) that lets you log on to your IM network from any Internet browser, with no firewall issues. Meebo supports AOL, Yahoo, MSN, ICQ, Google Talk and Jabber IM programs—you just go to its web site and log on to your network of choice.
Even better, Meebo runs pretty much like your actual IM program would. For those with lots of time, friends or ADD, you can log onto multiple accounts at once (or create a free Meebo account that remembers all your IM accounts for you). It's easy to use and constantly being upgraded with new features. We didn't find a firewall that could stop it.
We're not sure if avoiding your company's firewall truly counts as sticking it to The Man, but at least it's one less way The Man is sticking it to you.