Trillian is a powerful instant messaging program designed for various platforms that allows you to chat with your Facebook, Yahoo, and Gtalk friends from a single app. This utility provides you with numerous features, is simple to handle and configure, and syncs data with Trillian for iPhone for when you're away from your computer.
Before using this app you need to create a Trillian account (you can do it directly from the Setup window of the app or go to the homepage of the developer) and validate it. Afterwards, you can start working with the program. The app lets you log in to Gtalk, Facebook, AIM, Yahoo! or Twitter accounts, talk with your friends, and much more.
Trillian also features an integrated spell checker, allows you to customize chat settings according to your needs (change font style and size, display emoticons in messages, etc.), and notifies you when a new conversation is started, etc.
Sadly, this version of Trillian doesn't offer you any support for Skype or LinkedIn services. However, when it came to logging to my other accounts (Gtalk, Facebook, and Yahoo!), the application worked flawlessly.
To summarize, I'd say it's worth trying this program on your Mac as long as the aforementioned flaw doesn't matter too much to you. Trillian brings you a straightforward interface, displays mail alerts when it's the case, and provides you with a full-featured message history viewer.