Pidgin is one of the many multi-protocol instant messengers. And its available for many plattforms (Windows, GNU/Linux, Mac OS X, etc). Although pidgin supports many protocols out-of-the-box (f. ex. ICQ, AIM, IRC, XMPP/Jabber, Groupwise), new plugins are constantly emerging. The Pidgin-Plugin page gives a nice summary.
Some additional plugins I’ve come across are:
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Skype Note: This still requires a working Skype installation (http://eion.robbmob.com/)
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Twitter Supports Twitter, Identi.ca and Laconica-base servers. You can microblog directly from within pidgin. (http://code.google.com/p/microblog-purple/)