Start iChat/Adium without logging in
If you use iChat or Adium and have multiple accounts (AOL, Yahoo, Gtalk), you probably have the program set to automatically log into all accounts when it starts up. But there might be times when you want to log into just one account or none at all.
All you need to do is hold down the shift key while opening your chat program. Once the application launches you can choose which accounts you want to log into.