There are a lot of instant messaging services out there. There are fewer instant messaging clients that are actually capable of supporting multiple services and protocols. Here are the best for the Mac.
Adium is a free app for the Mac that can connect you with quite a few different instant messaging services. It works with AOL Instant Messenger, XMPP (so it works with Google Talk and Facebook Chat), MSN Messenger, ICQ, Twitter, IRC, and more.
Adium supports features like tabbed windows, integrates support for OS X's own Contacts apps, and lets you theme your message windows so you can customize the look and feel. It's a nice alternative to Apple's own Messages application, especially if you're dependent on instant messaging systems that Messages doesn't support.
- Free - Download now
Apple's default messaging app is a vital tool, especially thanks to its integration with Apple's iMessage ecosystem, making it possible to trade messages, audio and video with anyone who uses a Mac or an iOS device. It supports other messaging protocols, too: AOL Instant Messenger, Google Talk, Jabber and Yahoo. For many people, Messages is the only instant messaging app they need.
Where Messages comes up short is in supporting protocols like IRC, ICQ and some others. That's where Adium comes to the rescue.
Like Adium, Trillian supports a broad number of protocols: Google Talk, MSN, AIM, ICQ, Facebook, Yahoo and others. It does a great job of integrating with social networks. Trillian's also available for iOS, so you can have the same messaging experience regardless of what device you're using.
Trillian is free to download, but to get the most out of it you'll have to pay. It's available for a one-time charge of $60, or a 20/year "Pro" subscription, both of which net you lots of other useful features like cloud history, tabbed chats and integration with email, so you can do basic mail management or just get alerted to new messages as they come in.
- Free - Download now
The bottom line
I've left off other applications that people certainly use for instant messaging, but don't fit the multiprotocol mode. Skype is used by hundreds of millions, for example, but it's not designed to support other systems besides Skype. I've also avoided IM systems that depend on a browser to work.
But still I imagine I might have left some out. If there are Mac instant messaging apps that you can't live without, give 'em a shout out in the comments!