Pidgin

OK, for most everyone this is a no brainer. Pidgin according to their website:

Pidgin is a chat program which lets you log in to accounts on multiple chat networks simultaneously. This means that you can be chatting with friends on MSN, talking to a friend on Google Talk, and sitting in a Yahoo chat room all at the same time.

I use pidgin every day as part of my work communication. We have several sites spread all over the state and we can communicate instantly and painlessly. The best aspect of this mode of communication for me is that it can be asynchronous unlike the telephone. When the telephone rings I have to stop what I am doing, whether it is teaching a class or working on a computer; conversely, if someone sends me an IM I can get the message and reply when it is convenient for me.

Pidgin supports several networks and you can use them all simultaneously and seamlessly. Typically, I am logged in to Twitter, AIM, Yahoo and occasionally IRC but one can also use Myspace, gadu gadu, MSN, Google or several others.
One feature that most people don't know about is the ability to send messages to mobile phones from Pidgin. You just select start a new conversation and type in +########## where the #s would be the phone number including the area code.

Of course Pidgin is free and open source or I wouldn't be mentioning it here as you well know by now...