Adium is a popular and, probably, the best instant messaging client for you Mac at the moment. It supports a great number of services, including MSN, Yahoo! and AOL Instant Messenger, Google and Live Journal Talk, Facebook Chat, Twitter, ICQ, Bonjour, etc. Moreover, the application offers you a variety of options and high customizability; you will be able to adjust almost everything from the app's appearance to file transfer parameters.
Among the numerous features of the application, I would emphasize Tabbed Messaging and Encrypted Messaging. Adium allows you to combine multiple chats into a number of windows with tabs, which make your work more convenient and gives you more free screen volume. You can enable the option in General Preferences. Also, there you are able to set the program organize tabs into new windows by group. Due to the Off-the-Record Messaging support Adium allows you to send encrypted messages. Therefore you can be sure that no one will read your private message but your recipient and you.
In addition to all above, Adium gratifies you with cute interface and offers a lot of 'Xtras': sounds, emoticons, images, styles and many more. By the way, it is translated into 27 languages and some more localizations are in progress.
Unfortunately, Adium does not support voice and video chat, which, to my mind, is the only disadvantage of the program.