I use the excellent IM client Adium to connect to Google Talk, AIM and MSN. Today Adium ran into issues while connecting to Google Talk and MSN from the JHU network, probably because it was trying to use the default port 5222, which is blocked by the Hopkins firewall. After changing the options to those below, I was able to connect just fine.

Google Talk (from this Adium bug tracker entry):

• Change port to 443
• Enable "Allow plaintext authentication" and "Force old-style SSL"

MSN:

• Enable "Connect via HTTP"