Pidgin – http://www.pidgin.im Formerly known as GAIM, this product allows you to connect to a variety of instant messaging systems so that you can have all of your IM contacts in one place. It eliminates the need to open multiple applications to communicate with all of your friends and associates. It is lacking some of the functionality of the proprietary IM systems such as Windows Live Messenger, but for pure communications, it make for easier organizing of your contact lists.
Trillian – http://www.trillian.cc/ Trillian is another fine instant messaging application. There is a commercial offering for the Pro version, but the free version should work for most users. It allows you to integrate not only all of your IM clients, but ICQ and IRC as well so chat and IM become one application. There is currently a beta program for the next version named Astra. It can take months to be invited to test the beta, but from appearances, it should be worth the wait. I have not used the application myself, but I know of others that are more than pleased with their testing.