If you don’t already know about Adium, then you’re missing out.
Adium is an instant messaging client that permits you to use a single program to communicate with people on virtually every instant messaging service in the world. So you don’t have to worry about whether all your friends and colleagues are on MSN, or Yahoo, or AIM, or iChat, or Google Talk, or Skype, or, well, you get the idea.
That, alone, is a neat trick, but it’s hardly unique. There are other programs (such as the excellent Fire) which do the same thing.
Adium raises the bar in how you can communicate over each of these networks.
Not only can you customize the look of the application to the Nth degree, but you can also automate it (send a message to a buddy as soon as they log in, for example), secure it (encrypt your conversations quickly and easily between Adium users), and track it (incredibly comprehensive logging, including log searching).
Additionally, Adium supports a few services that I haven’t found on any other Mac clients, such as Lotus Sametime (which I have to use at work and has no Mac client otherwise – not even from IBM/Lotus).
What it cannot do (and makes no effort to attempt) is video or audio chatting. This is purely for text-based IM’ing. However, nothing prevents you from running Adium and then opening up iChat when you need to video conference.
Great program, and did I mention that it’s 100% free?