Instant messaging is an essential part of modern communications. On the Mac, we're blessed with the excellent iChat, but Apple's built-in IM solution doesn't connect to every protocol, so that's when we turn to Adium.
Adium is a great long-standing piece of open-source Mac software. The reason it's so good is that it connects to almost every IM protocol under the sun. Gtalk, Facebook IM, Live Messenger, Yahoo, AIM, Jabber, IRC, you name it, Adium will let you chat using it.
The Swiss Army Knife of chat clients, Adium not only lets you connect and chat, but does it in a very customizable, yet Mac-like way. You've got Growl support, plugins, a menu bar icon, sounds, notifications and alerts for every action and a highly recognizable icon, even if it doesn't scream "chat" to you at first glance.
Setting up Adium couldn't be easier. Select the desired protocol, plug in your login details and you're good to go. Your buddy list is downloaded from the respective sites and blended together in one list. You can sort by protocol or create your own groupings. Cross-protocol chat isn't possible for those that don't have interoperability, but that's no fault of the chat client. Adium is purely text-based, there are no built-in options for video or audio chat at the moment, but you can send files for protocols that support file transfer.
Adium is available for free from the Adium team's site, and works great on almost all versions of OS X including Lion. If you've got friends spread across a plethora of IM protocols, let Adium simplify things and get you chatting to them all from one great client.