I wrote recently about sites that bring your social networks together. This makes it easier for my friends to see what I'm up to. And saving time for me is why I've brought together all my e-mail accounts into GMail.
So why should I run several Instant Messaging programs when I can combine all in one program? Pidgin http://www.pidgin.im/ does just that, allowing multiple IM accounts such as Yahoo, Hotmail, Gmail chat, along with the chat now available in Facebook. It's also good at notifying you when mail arrives in one of your various accounts, and can be extended by means of a host of useful plug-ins.
UPDATE: the latest version of Pidgin continually froze when exiting. So now I use Digsby.com!