Chat on AIM/iChat from behind a firewall
It is not unusual for companies and schools to block certain ports to prevent people from using things like P2P networks and AIM. If you want to get around the AIM port block, change the port from the default 5190 to port 80 (the standard for web servers), which is unlikely to be blocked. Accessing this preference is easy, in iChat navigate to Preferences->Accounts->Server Settings, and adjust the settings as shown in the picture below. Now you can talk on iChat or AIM from school or work!