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Nik's Pick: Adium
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.
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